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Short-Term Debt And Liquidity Short-Term Debt And Liquidity (Commercial Paper) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Commercial Paper    
Line of Credit Facility [Line Items]    
Average Daily Commercial Paper Borrowings Outstanding $ 767 $ 639
Commercial paper outstanding $ 301 $ 714
Weighted average interest rate 0.55% 0.36%
Peak short-term borrowings [1] $ 1,108 $ 910
Peak short-term borrowings interest rate 0.91% 0.75%
Parent Company    
Line of Credit Facility [Line Items]    
Average Daily Commercial Paper Borrowings Outstanding $ 721 $ 423
Weighted average interest rate 0.57% 0.36%
Peak short-term borrowings [1] $ 874 $ 625
Peak short-term borrowings interest rate 0.91% 0.75%
Parent Company | Commercial Paper    
Line of Credit Facility [Line Items]    
Commercial paper outstanding $ 301 $ 585
Union Electric Company    
Line of Credit Facility [Line Items]    
Average Daily Commercial Paper Borrowings Outstanding $ 42 $ 110
Weighted average interest rate 0.50% 0.38%
Peak short-term borrowings [1] $ 294 $ 495
Peak short-term borrowings interest rate 0.60% 0.70%
Ameren Illinois Company    
Line of Credit Facility [Line Items]    
Average Daily Commercial Paper Borrowings Outstanding $ 4 $ 165
Weighted average interest rate 0.44% 0.32%
Peak short-term borrowings [1] $ 48 $ 300
Peak short-term borrowings interest rate 0.60% 0.60%
Ameren Illinois Company | Commercial Paper    
Line of Credit Facility [Line Items]    
Commercial paper outstanding $ 0 $ 32
Ameren Missouri [Member] | Commercial Paper    
Line of Credit Facility [Line Items]    
Commercial paper outstanding $ 0 $ 97
[1] (a)The timing of peak commercial paper issuances varies by company, and therefore the peak amounts presented by company might not equal the Ameren Consolidated peak commercial paper issuances for the period.