Short-Term Debt And Liquidity (Narrative) (Detail) $ in Billions	3 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019
Credit Agreements
Short-term Debt [Line Items]
Net Liquidity Available	$ 1.7
Actual debt-to-capital ratio	0.55
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	1.93%	2.87%
Union Electric Company | Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.51
Ameren Illinois Company | Illinois Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.46