Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	$ 271	$ 235
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	22	(111)
Issuance of common stock for stock-based compensation	33	38
Nuclear Fuel [Member]
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	0	7
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	141	97
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	22	(111)
Issuance of common stock for stock-based compensation	0	0
Union Electric Company | Nuclear Fuel [Member]
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	0	7
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	139	127
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	0	0
Issuance of common stock for stock-based compensation	0	0
Ameren Illinois Company | Nuclear Fuel [Member]
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	$ 0	$ 0