Asset Retirement Obligation By Type (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Asset Retirement Obligations By Type [Line Items]
Regulatory Liabilities	$ 1,813		$ 1,731
Total Asset Retirement Obligation Liability	490		470		390
Quad Cities Station [Member]
Asset Retirement Obligations By Type [Line Items]
Total Asset Retirement Obligation Liability	243		230
Fossil fuel facilities [Member]
Asset Retirement Obligations By Type [Line Items]
Total Asset Retirement Obligation Liability	123		114
Wind generating facilities [Member]
Asset Retirement Obligations By Type [Line Items]
Total Asset Retirement Obligation Liability	61		41
Offshore pipeline facilities [Member]
Asset Retirement Obligations By Type [Line Items]
Total Asset Retirement Obligation Liability	36		50
Other [Member]
Asset Retirement Obligations By Type [Line Items]
Total Asset Retirement Obligation Liability	27		35
Environmental Restoration Costs [Member]
Asset Retirement Obligations By Type [Line Items]
Regulatory Liabilities	1,461	[1]	1,404	[1]
Quad Cities Station nuclear decommissioning trust funds [Member]
Asset Retirement Obligations By Type [Line Items]
Nuclear decommissioning trust funds	$ 337		$ 306

[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.