Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	$ 434	$ 287
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	85	(4)
Issuance of common stock for stock-based compensation	33	38
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	259	111
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	85	(4)
Issuance of common stock for stock-based compensation	0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures, including wind generation expenditures	174	165
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	0	0
Issuance of common stock for stock-based compensation	$ 0	$ 0