Property And Plant, Net (Schedule Of Property And Plant, Net) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) equipment	Dec. 31, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	$ 32,027	$ 30,118
Accumulated depreciation and amortization	11,633	10,961
Property and plant, before construction work in progress	20,394	19,157
Property, Plant and Equipment, Net	$ 21,466	20,113
Number of combustion turbine electric generation equipment under capital lease agreements | equipment	2
Number of capital lease agreements | equipment	2
Capital lease agreements, gross asset value	$ 233	232
Total accumulated depreciation, capital lease agreements	83	77
Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	11,132	10,911
Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	11,415	10,850
Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	4,666	3,879
Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,893	2,641
Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,921	1,837
Construction work in progress	924	750
Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	148	206
Union Electric Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	19,495	18,959
Accumulated depreciation and amortization	8,305	7,880
Property and plant, before construction work in progress	11,190	11,079
Property, Plant and Equipment, Net	11,751	11,478
Union Electric Company | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	11,132	10,911
Union Electric Company | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	5,766	5,563
Union Electric Company | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,201	1,151
Union Electric Company | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	474	455
Union Electric Company | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	922	879
Construction work in progress	413	193
Union Electric Company | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	148	206
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	11,123	10,208
Accumulated depreciation and amortization	3,082	2,850
Property and plant, before construction work in progress	8,041	7,358
Property, Plant and Equipment, Net	8,293	7,469
Ameren Illinois Company | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Ameren Illinois Company | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	5,649	5,287
Ameren Illinois Company | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,298	2,016
Ameren Illinois Company | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,419	2,186
Ameren Illinois Company | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	757	719
Construction work in progress	252	111
Ameren Illinois Company | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	0	0
Other
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,409	951
Accumulated depreciation and amortization	246	231
Property and plant, before construction work in progress	1,163	720
Property, Plant and Equipment, Net	1,422	1,166
Other | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Other | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Other | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,167	712
Other | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Other | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	242	239
Construction work in progress	259	446
Other | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	$ 0	$ 0
Minimum | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Minimum | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Minimum | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	50 years
Minimum | Gas
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Minimum | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Maximum | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	72 years
Maximum | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	80 years
Maximum | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	75 years
Maximum | Gas
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	80 years
Maximum | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	55 years
Maximum | Union Electric Company | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	150 years