Short-Term Debt And Liquidity (Narrative) (Detail) $ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016	Sep. 30, 2017 USD ($)	Sep. 30, 2016
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	1.24%	0.53%	1.18%	0.54%
Credit Agreements
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1.7		$ 1.7
Actual debt-to-capital ratio	0.51		0.51
Missouri Credit Agreement | Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.47		0.47
Illinois Credit Agreement | Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.46		0.46