Asset Retirement Obligations - Asset Retirement Obligation By Type (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		$ 954	$ 921	$ 753
Regulatory Liabilities		3,120	3,033
Fossil Fuel Plant [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		404	443
Quad Cities Station [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		343	289
Wind Generating Facility [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		124	104
Offshore pipeline facilities [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		33	31
Solar Generating Facility [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		12	12
Other Plant in Service [Member]
Asset Retirement Obligations By Type [Line Items]
Asset Retirement Obligation		38	42
Removal Costs [Member]
Asset Retirement Obligations By Type [Line Items]
Regulatory Liabilities	[1]	2,242	2,167
Quad Cities Station nuclear decommissioning trust funds [Member]
Asset Retirement Obligations By Type [Line Items]
Nuclear decommissioning trust funds		$ 460	$ 429

[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.