Short-Term Debt And Liquidity (Narrative) (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	1.90%	1.01%
Credit Agreements
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,200,000,000.0
Actual debt-to-capital ratio	0.54
Missouri Credit Agreement | Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.50
Illinois Credit Agreement | Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.48