Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 736	$ 417
Weighted Average Interest Rate		1.06%	0.81%
Peak Short Term Borrowings	[1]	$ 914	$ 581
Peak Short Term Borrowings Interest Rate		1.30%	0.95%
Ameren (parent)
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 682	$ 349
Weighted Average Interest Rate		1.07%	0.82%
Peak Short Term Borrowings	[1]	$ 810	$ 482
Peak Short Term Borrowings Interest Rate		1.30%	0.95%
Union Electric Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 4	$ 68
Weighted Average Interest Rate		0.93%	0.80%
Peak Short Term Borrowings	[1]	$ 45	$ 208
Peak Short Term Borrowings Interest Rate		1.15%	0.85%
Ameren Illinois Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 50	$ 0
Weighted Average Interest Rate		0.95%	0.00%
Peak Short Term Borrowings	[1]	$ 74	$ 0
Peak Short Term Borrowings Interest Rate		1.15%	0.00%

[1] (a)The timing of peak commercial paper issuances varies by company. Therefore, the sum of peak commercial paper issuances presented by company does not equal the Ameren Consolidated peak commercial paper issuances for the period.