Short-Term Debt And Liquidity (Narrative) (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017	Sep. 30, 2018 USD ($)	Sep. 30, 2017
Credit Agreements
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,578,000,000		$ 1,578,000,000
Actual debt-to-capital ratio	0.52		0.52
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	2.00%	1.24%	2.02%	1.18%
Union Electric Company | Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.46		0.46
Ameren Illinois Company | Illinois Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.48		0.48