Short-Term Debt And Liquidity (Narrative) (Detail) $ in Billions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016	Jun. 30, 2017 USD ($)	Jun. 30, 2016
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	1.27%	0.60%	1.14%	0.54%
Credit Agreements
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1.2		$ 1.2
Actual debt-to-capital ratio	0.53		0.53
Missouri Credit Agreement | Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.48		0.48
Illinois Credit Agreement | Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.47		0.47