Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 531	$ 843
Weighted Average Interest Rate		0.80%	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		$ 402	$ 754
Weighted Average Interest Rate		0.82%	0.57%
Peak Short Term Borrowings	[1]	$ 549	$ 849
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		$ 117	$ 84
Weighted Average Interest Rate		0.74%	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		$ 12	$ 5
Weighted Average Interest Rate		0.79%	0.44%
Peak Short Term Borrowings	[1]	$ 177	$ 39
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.