Asset Retirement Obligations - Asset Retirement Obligation By Type (Details) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		$ 921	$ 753	$ 696
Regulatory Liabilities		3,033	2,832
Fossil Fuel Plant [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		443	334
Quad Cities Station [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		289	265
Wind Generating Facility [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		104	75
Offshore pipeline facilities [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		31	31
Solar Generating Facility [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		12	9
Other Plant in Service [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		42	39
Removal Costs [Member]
Asset Retirement Obligations By Type [Line Items]
Regulatory Liabilities	[1]	2,167	2,215
Quad Cities Station nuclear decommissioning trust funds [Member]
Asset Retirement Obligations By Type [Line Items]
Nuclear decommissioning trust funds		$ 429	$ 424

[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.