Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 563	$ 832
Weighted Average Interest Rate		0.793%	0.56%
Peak Short Term Borrowings	[1]	$ 839	$ 1,108
Peak Short Term Borrowings Interest Rate		0.95%	0.70%
Ameren (parent)
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 435	$ 770
Weighted Average Interest Rate		0.811%	0.56%
Peak Short Term Borrowings	[1]	$ 574	$ 874
Peak Short Term Borrowings Interest Rate		0.95%	0.70%
Union Electric Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 80	$ 56
Weighted Average Interest Rate		0.739%	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		$ 48	$ 6
Weighted Average Interest Rate		0.722%	0.44%
Peak Short Term Borrowings	[1]	$ 195	$ 48
Peak Short Term Borrowings Interest Rate		0.85%	0.60%

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