v3.8.0.1
RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - LEVEL 3 INPUTS (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2018
CAD ($)
$ / bbl
$ / MWh
$ / MillionsofBTU-MMBTU
$ / Gallon-gal
Dec. 31, 2017
CAD ($)
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Fair Value $ (2,359) $ (2,192)
Recurring basis    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Fair Value (2,359) (2,192)
Level 3 | Recurring basis    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Fair Value (205) $ (387)
Market approach valuation technique | Recurring basis | Commodity Contracts - Financial | NGL    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Fair Value $ (4)  
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Natural gas | Minimum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / MillionsofBTU-MMBTU 2.49  
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Natural gas | Maximum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / MillionsofBTU-MMBTU 4.25  
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Natural gas | Weighted Average    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / MillionsofBTU-MMBTU 3.20  
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Crude oil | Minimum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / bbl 48.92  
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Crude oil | Maximum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / bbl 63.73  
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Crude oil | Weighted Average    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / bbl 53.07  
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | NGL | Minimum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / Gallon-gal 0.34  
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | NGL | Maximum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / Gallon-gal 1.83  
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | NGL | Weighted Average    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / Gallon-gal 1.29  
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Power | Minimum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / MWh 14.30  
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Power | Maximum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / MWh 76.27  
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Power | Weighted Average    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / MWh 52.00  
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Natural gas | Minimum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / MillionsofBTU-MMBTU 0.78  
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Natural gas | Maximum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / MillionsofBTU-MMBTU 4.91  
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Natural gas | Weighted Average    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / MillionsofBTU-MMBTU 2.57  
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Crude oil | Minimum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / bbl 38.01  
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Crude oil | Maximum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / bbl 91.27  
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Crude oil | Weighted Average    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / bbl 75.29  
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | NGL | Minimum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / Gallon-gal 0.34  
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | NGL | Maximum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / Gallon-gal 1.88  
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | NGL | Weighted Average    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Forward price | $ / Gallon-gal 0.86  
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Financial | Natural gas    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Fair Value $ 9  
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Financial | Crude oil    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Fair Value (4)  
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Financial | Power    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Fair Value (100)  
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Physical | Natural gas    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Fair Value (81)  
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Physical | Crude oil    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Fair Value (29)  
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Physical | NGL    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Fair Value $ 5  
Option model valuation technique | Level 3 | Commodity Options | Crude oil | Minimum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Option Volatility (as a percent) 22.00%  
Option model valuation technique | Level 3 | Commodity Options | Crude oil | Maximum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Option Volatility (as a percent) 24.00%  
Option model valuation technique | Level 3 | Commodity Options | Crude oil | Weighted Average    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Option Volatility (as a percent) 23.00%  
Option model valuation technique | Level 3 | Commodity Options | NGL | Minimum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Option Volatility (as a percent) 0.00%  
Option model valuation technique | Level 3 | Commodity Options | NGL | Maximum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Option Volatility (as a percent) 0.00%  
Option model valuation technique | Level 3 | Commodity Options | NGL | Weighted Average    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Option Volatility (as a percent) 0.00%  
Option model valuation technique | Level 3 | Commodity Options | Power | Minimum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Option Volatility (as a percent) 23.00%  
Option model valuation technique | Level 3 | Commodity Options | Power | Maximum    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Option Volatility (as a percent) 26.00%  
Option model valuation technique | Level 3 | Commodity Options | Power | Weighted Average    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Option Volatility (as a percent) 24.00%  
Option model valuation technique | Level 3 | Recurring basis | Commodity Options | Crude oil    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Fair Value $ (1)  
Option model valuation technique | Level 3 | Recurring basis | Commodity Options | Power    
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments    
Fair Value $ 0