Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 808	$ 531
Weighted-average interest rate		1.19%	0.80%
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		$ 736	$ 402
Weighted-average interest rate		1.19%	0.82%
Peak commercial paper during period	[1]	$ 841	$ 549
Peak interest rate		1.50%	0.95%
Union Electric Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 6	$ 117
Weighted-average interest rate		1.10%	0.74%
Peak commercial paper during period	[1]	$ 60	$ 208
Peak interest rate		1.41%	0.85%
Ameren Illinois Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 66	$ 12
Weighted-average interest rate		1.14%	0.79%
Peak commercial paper during period	[1]	$ 163	$ 177
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.