Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Supplemental Cash Flow Information [Line Items]
Exchange of bond investments for extinguishment of senior unsecured notes	$ 17	$ 0
Accrued Capital Expenditures	273	240
Net Realized and Unrealized Gain (Loss) - nuclear decommissioning trust fund	100	33
Noncash financing activity – Issuance of common stock for stock-based compensation	54	35
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Exchange of bond investments for extinguishment of senior unsecured notes	0	0
Accrued Capital Expenditures	138	94
Net Realized and Unrealized Gain (Loss) - nuclear decommissioning trust fund	100	33
Noncash financing activity – Issuance of common stock for stock-based compensation	0	0
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Exchange of bond investments for extinguishment of senior unsecured notes	17	0
Accrued Capital Expenditures	128	133
Net Realized and Unrealized Gain (Loss) - nuclear decommissioning trust fund	0	0
Noncash financing activity – Issuance of common stock for stock-based compensation	$ 0	$ 0