Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	$ 508	$ 446	$ 333
Net realized and unrealized gain – nuclear decommissioning trust fund	163	116	143
Exchange of bond investments for extinguishment of senior unsecured notes	0	0	(17)
Noncash financing activity – Issuance of common stock for stock-based compensation	33	38	54
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	285	229	140
Net realized and unrealized gain – nuclear decommissioning trust fund	163	116	143
Exchange of bond investments for extinguishment of senior unsecured notes	0	0	0
Noncash financing activity – Issuance of common stock for stock-based compensation	0	0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	215	218	163
Net realized and unrealized gain – nuclear decommissioning trust fund	0	0	0
Exchange of bond investments for extinguishment of senior unsecured notes	0	0	(17)
Noncash financing activity – Issuance of common stock for stock-based compensation	0	0	0
Ameren Illinois Company | Senior Unsecured 1993 Series B-1 [Domain] | Unsecured Debt
Supplemental Cash Flow Information [Line Items]
Debt instrument face amount			17
Nuclear Fuel
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	16	0	19
Nuclear Fuel | Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	16	0	19
Nuclear Fuel | Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	$ 0	$ 0	$ 0