Short-Term Debt And Liquidity (Narrative) (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017	Jun. 30, 2018 USD ($)	Jun. 30, 2017
Credit Agreements
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.54		0.54
Line of credit facility, maximum borrowing capacity	$ 1,600,000,000.0		$ 1,600,000,000.0
Utilities [Member]
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	2.17%	1.27%	2.04%	1.14%
Union Electric Company | Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.48		0.48
Ameren Illinois Company | Illinois Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.47		0.47