Variable Interest Entities (Details) (Lower Mount Bethel Energy, LLC [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Lower Mount Bethel Energy, LLC [Member]
Variable Interest Entities [Line Items]
Original acquisition cost of the facility owned by a VIE	$ 455
Residual value guarantee percentage associated with VIE lease	70.52%
Long-term debt associated with VIE	437	437
Non-controlling interests associated with VIE	18	18
Additional depreciation expense for leasehold improvements associated with VIE	$ 16	$ 16	$ 11