Fair Value Measurements Nonrecurring Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011 MW	Sep. 30, 2012	Sep. 30, 2011 MW
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of property, plant, and equipment prior to impairment charge		$ 79		$ 79
Property, Plant, and Equipment, Fair Value Disclosure		28		28
Impairment of Long-Lived Assets Held-for-use	0	0	0	51
Impairment Of Long Lived Assets Held For Use After Tax				31
Megawatts associated with natural gas plants held for sale		2,152		2,152
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down	0	148	0	148
Loss on assets held for sale, after-tax		97		97
Gain (Loss) on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring, Net		(21)		(21)
Assets Held For Sale, Income Statement Location [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (Loss) on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring, Net		$ (30)		$ (30)