Short-Term Debt And Liquidity Short-Term Debt And Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Commercial Paper
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 552	$ 767
Commercial paper outstanding		$ 558	$ 301
Weighted average interest rate		0.80%	0.55%
Peak short-term borrowings	[1]	$ 839	$ 1,108
Peak short-term borrowings interest rate		1.05%	0.91%
Parent Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 440	$ 721
Weighted average interest rate		0.82%	0.57%
Peak short-term borrowings	[1]	$ 574	$ 874
Peak short-term borrowings interest rate		1.05%	0.91%
Parent Company | Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding		$ 507	$ 301
Union Electric Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 60	$ 42
Weighted average interest rate		0.74%	0.50%
Peak short-term borrowings	[1]	$ 208	$ 294
Peak short-term borrowings interest rate		0.85%	0.60%
Ameren Illinois Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 52	$ 4
Weighted average interest rate		0.69%	0.44%
Peak short-term borrowings	[1]	$ 195	$ 48
Peak short-term borrowings interest rate		0.90%	0.60%
Ameren Illinois Company | Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding		$ 51	$ 0
Ameren Missouri [Member] | Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding		$ 0	$ 0

[1] (a)The timing of peak outstanding commercial paper issuances varies by company. Therefore, the sum of the peak amounts presented by company might not equal the Ameren Consolidated peak amount for the period.