Short-Term Debt And Liquidity (Narrative) (Detail) $ in Billions	3 Months Ended
	Mar. 31, 2017 USD ($)	Mar. 31, 2016
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	1.01%	0.47%
Credit Agreements 2012
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1.2
Actual debt-to-capital ratio	0.53
Missouri Credit Agreement 2012 | Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.48
Illinois Credit Agreement 2012 | Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.46