Financial Instruments, Derivatives, Guarantees and Concentration of Credit Risk (Details 2) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Fair value of energy commodity derivatives
Total derivatives, assets	$ 401	$ 573
Total derivatives, liabilities	216	289
Legacy natural gas contracts from former power business | Not Designated as Hedging Instrument
Fair value of energy commodity derivatives
Total derivatives, assets	174	186
Total derivatives, liabilities	173	187
Other Contract | Not Designated as Hedging Instrument
Fair value of energy commodity derivatives
Total derivatives, assets	18	99
Total derivatives, liabilities	16	80
Designated as Hedging Instrument
Fair value of energy commodity derivatives
Total derivatives, assets	209	288
Total derivatives, liabilities	27	22
Not Designated as Hedging Instrument
Fair value of energy commodity derivatives
Total derivatives, assets	192	285
Total derivatives, liabilities	$ 189	$ 267