Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplemental Cash Flow Information [Line Items]
Exchange of bond investments for extinguishment of senior unsecured notes	$ 17	$ 0	$ 0
Accrued capital expenditures	333	272	361
Net Realized and Unrealized Gain (Loss) - nuclear trust fund	143	(38)	3
Noncash financing activity - Issuance of common stock for stock-based compensation	54	35	0
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Exchange of bond investments for extinguishment of senior unsecured notes	0	0	0
Accrued capital expenditures	140	121	159
Net Realized and Unrealized Gain (Loss) - nuclear trust fund	143	(38)	3
Noncash financing activity - Issuance of common stock for stock-based compensation	0	0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Exchange of bond investments for extinguishment of senior unsecured notes	17	0	0
Accrued capital expenditures	163	138	175
Net Realized and Unrealized Gain (Loss) - nuclear trust fund	0	0	0
Noncash financing activity - Issuance of common stock for stock-based compensation	0	0	0
Nuclear Fuel
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	19	20	10
Nuclear Fuel | Union Electric Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	19	20	10
Nuclear Fuel | Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Accrued capital expenditures	$ 0	$ 0	$ 0