Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	$ 235	$ 208
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	(111)	64
Issuance of common stock for stock-based compensation	38	54
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	97	92
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	(111)	64
Issuance of common stock for stock-based compensation	0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	127	106
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	0	0
Issuance of common stock for stock-based compensation	0	0
Nuclear Fuel
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	7	0
Nuclear Fuel | Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	7	0
Nuclear Fuel | Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	$ 0	$ 0