Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures		$ 311	$ 273
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund		43	100
Issuance of common stock for stock-based compensation	$ 38	38	54
Debt Conversion, Original Debt, Amount		0	(17)
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures		115	138
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund		43	100
Issuance of common stock for stock-based compensation		0	0
Debt Conversion, Original Debt, Amount		0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures		191	128
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund		0	0
Issuance of common stock for stock-based compensation		0	0
Debt Conversion, Original Debt, Amount		$ 0	$ (17)