Divestitures (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Divestitures [Abstract]
Proceeds from Sales of Producing Properties and Acreage	$ 1,300	$ 1,400	$ 673
Assets Held-for-sale, at Carrying Value, Total	310
Liabilities of Assets Held-for-sale	31
Proceeds received on sale of Canadian shallow gas assets			344
Impairment recorded to adjust Canadian shallow gas assets to fair value, less cost to sell			$ 280