v3.8.0.1
Long-Term Debt - Components of Long-Term Debt (Parenthetical) (Detail)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Feb. 16, 2016
Sep. 21, 2015
Feb. 11, 2015
Jul. 01, 2014
Mar. 12, 2013
6.125% Senior Notes Due 2021 [Member]              
Debt Instrument [Line Items]              
Debt instrument interest rate 6.125% 6.125%         6.125%
Senior notes maturity year 2021 2021          
5.125% Senior Notes Due 2022 [Member]              
Debt Instrument [Line Items]              
Debt instrument interest rate 5.125% 5.125%       5.125%  
Senior notes maturity year 2022 2022          
5.625% Senior Notes Due 2023 [Member]              
Debt Instrument [Line Items]              
Debt instrument interest rate 5.625% 5.625%   5.625% 5.625%    
Senior notes maturity year 2023 2023          
6.500% Senior Notes Due 2024 [Member]              
Debt Instrument [Line Items]              
Debt instrument interest rate 6.50% 6.50% 6.50%        
Senior notes maturity year 2024 2024          
9.0% Revenue Bonds [Member]              
Debt Instrument [Line Items]              
Debt instrument interest rate 9.00% 9.00%          
9.5% Revenue Bonds [Member]              
Debt Instrument [Line Items]              
Debt instrument interest rate 9.50% 9.50%