Short-Term Debt And Liquidity (Narrative) (Detail)	3 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018
Credit Agreements
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,292,000,000
Actual debt-to-capital ratio	0.54
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	2.87%	1.90%
Union Electric Company | Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.48
Ameren Illinois Company | Illinois Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.47