Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 852		$ 539
Weighted Average Interest Rate	0.53%		0.45%
Peak Short Term Borrowings	955	[1]	700	[1]
Peak Short Term Borrowings Interest Rate	0.70%		0.75%
Ameren (parent)
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	691		339
Weighted Average Interest Rate	0.55%		0.45%
Peak Short Term Borrowings	815	[1]	452	[1]
Peak Short Term Borrowings Interest Rate	0.70%		0.75%
Union Electric Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	151		200
Weighted Average Interest Rate	0.49%		0.45%
Peak Short Term Borrowings	243	[1]	290	[1]
Peak Short Term Borrowings Interest Rate	0.60%		0.70%
Ameren Illinois Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	10		0
Weighted Average Interest Rate	0.44%		0.00%
Peak Short Term Borrowings	$ 39	[1]	$ 0	[1]
Peak Short Term Borrowings Interest Rate	0.60%		0.00%

[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.