Property And Plant, Net (Accrued Capital Expenditures) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 361	$ 251	$ 235
Union Electric Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	159	116	85
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	175	87	92
Nuclear Fuel
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	10	20	16
Nuclear Fuel | Union Electric Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 10	$ 20	$ 16