Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 843	$ 607
Weighted Average Interest Rate		0.56%	0.32%
Peak Short Term Borrowings	[1]	$ 1,108	$ 907
Peak Short Term Borrowings Interest Rate		0.70%	0.70%
Ameren (parent)
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 754	$ 328
Weighted Average Interest Rate		0.57%	0.32%
Peak Short Term Borrowings	[1]	$ 849	$ 503
Peak Short Term Borrowings Interest Rate		0.70%	0.35%
Union Electric Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 84	$ 146
Weighted Average Interest Rate		0.50%	0.31%
Peak Short Term Borrowings	[1]	$ 294	$ 495
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		$ 5	$ 242
Weighted Average Interest Rate		0.44%	0.32%
Peak Short Term Borrowings	[1]	$ 39	$ 300
Peak Short Term Borrowings Interest Rate		0.60%	0.34%

[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.