Short-Term Debt And Liquidity (Narrative) (Detail) $ in Billions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019	Jun. 30, 2020 USD ($)	Jun. 30, 2019
Credit Agreements
Short-term Debt [Line Items]
Net Liquidity Available	$ 2.2		$ 2.2
Actual debt-to-capital ratio	0.56		0.56
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	0.42%	2.75%	1.18%	2.81%
Union Electric Company | Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.50		0.50
Ameren Illinois Company | Illinois Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.44		0.44