Short-Term Debt And Liquidity (Narrative) (Detail) $ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019	Sep. 30, 2020 USD ($)	Sep. 30, 2019
Credit Agreements
Short-term Debt [Line Items]
Net Liquidity Available	$ 2.0		$ 2.0
Actual debt-to-capital ratio	0.55		0.55
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	0.10%	2.40%	0.81%	2.67%
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	0.45		0.45