Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 832	$ 609
Weighted Average Interest Rate		0.56%	0.35%
Peak Short Term Borrowings	[1]	$ 1,108	$ 907
Peak Short Term Borrowings Interest Rate		0.70%	0.75%
Ameren (parent)
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 770	$ 386
Weighted Average Interest Rate		0.56%	0.36%
Peak Short Term Borrowings	[1]	$ 874	$ 531
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		$ 56	$ 141
Weighted Average Interest Rate		0.50%	0.38%
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		$ 6	$ 157
Weighted Average Interest Rate		0.44%	0.31%
Peak Short Term Borrowings	[1]	$ 48	$ 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.