Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 417	$ 852
Weighted Average Interest Rate		0.81%	0.53%
Peak Short Term Borrowings	[1]	$ 581	$ 955
Peak Short Term Borrowings Interest Rate		0.95%	0.70%
Ameren (parent)
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 349	$ 691
Weighted Average Interest Rate		0.82%	0.55%
Peak Short Term Borrowings	[1]	$ 482	$ 815
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		$ 68	$ 151
Weighted Average Interest Rate		0.80%	0.49%
Peak Short Term Borrowings	[1]	$ 208	$ 243
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		$ 0	$ 10
Weighted Average Interest Rate		0.00%	0.44%
Peak Short Term Borrowings	[1]	$ 0	$ 39
Peak Short Term Borrowings Interest Rate		0.00%	0.60%

[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.