Asset Retirement Obligations - Asset Retirement Obligation By Type (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		$ 954	$ 954	$ 921
Regulatory Liabilities		7,511	3,120
Fossil Fuel Plant [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		380	404
Quad Cities Station [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		342	343
Wind Generating Facility [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		138	124
Offshore pipeline facilities [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		32	33
Solar Generating Facility [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		19	12
Other Plant in Service [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		43	38
Removal Costs [Member]
Asset Retirement Obligations By Type [Line Items]
Regulatory Liabilities	[1]	2,349	2,242
Quad Cities Station nuclear decommissioning trust funds [Member]
Asset Retirement Obligations By Type [Line Items]
Nuclear decommissioning trust funds		$ 515	$ 460

[1] Amounts represent estimated costs, as accrued through depreciation rates and exclusive of ARO liabilities, of removing regulated property, plant and equipment in accordance with accepted regulatory practices. Amounts are deducted from rate base or otherwise accrue a carrying cost.