Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 754	$ 563
Weighted-average interest rate		1.27%	0.79%
Peak commercial paper during period	[1]	$ 948	$ 839
Peak interest rate		1.50%	0.95%
Ameren (parent)
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 669	$ 435
Weighted-average interest rate		1.27%	0.81%
Peak commercial paper during period	[1]	$ 841	$ 574
Peak interest rate		1.50%	0.95%
Union Electric Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 7	$ 80
Weighted-average interest rate		1.20%	0.74%
Peak commercial paper during period	[1]	$ 64	$ 208
Peak interest rate		1.41%	0.85%
Ameren Illinois Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 78	$ 48
Weighted-average interest rate		1.28%	0.72%
Peak commercial paper during period	[1]	$ 193	$ 195
Peak interest rate		1.50%	0.85%

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