Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	$ 287	$ 263
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	(4)	90
Issuance of common stock for stock-based compensation	38	54
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	111	101
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	(4)	90
Issuance of common stock for stock-based compensation	0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	165	143
Net realized and unrealized gain (loss) – nuclear decommissioning trust fund	0	0
Issuance of common stock for stock-based compensation	$ 0	$ 0