Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenues	$ 962	$ 928	$ 1,035	$ 1,113	$ 907	$ 868	$ 862	$ 734	$ 4,038	$ 3,371	$ 2,730
Operating income (loss)	285	286	367	435	317	309	325	252	1,373	1,204	(1,101)
Other Income (Loss), Net									(4)	2	1
Gain (loss) on derivatives									139	26	(15)
Depreciation, depletion and amortization									942	787	811
Impairment of natural gas and oil properties											1,940
Interest expense									59	42	35
Provision (benefit) for income taxes									525	486	(443)
Assets	14,925				8,048				14,925	8,048	6,738
Capital investments									7,447	2,235	2,081
Impairment of natural gas and oil properties											1,940
Change in accrued expenditures									155	(25)	(37)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									(3,182)	(2,380)	(1,600)
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenues									2,850	2,398	1,965
Operating income (loss)									1,013	879	(1,396)
Other Income (Loss), Net									(3)	3	(1)
Gain (loss) on derivatives									142	26	(15)
Depreciation, depletion and amortization									884	735	766
Impairment of natural gas and oil properties											1,940
Interest expense									47	30	20
Provision (benefit) for income taxes									402	368	(549)
Assets	13,018				6,357				13,018	6,357	5,194
Capital investments									7,254	2,052	1,861
Exploration and Production [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									(12)	(6)	1
Exploration and Production [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Assets	77				79				77	79	44
Capital investments									11	35	12
Midstream Services [Member]
Segment Reporting Information [Line Items]
Revenues									1,188	973	765
Operating income (loss)									361	325	294
Other Income (Loss), Net									(1)
Gain (loss) on derivatives									(1)
Depreciation, depletion and amortization									58	51	44
Interest expense									12	11	14
Provision (benefit) for income taxes									123	119	105
Assets	1,554				1,427				1,554	1,427	1,273
Capital investments									144	158	165
Midstream Services [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									(3,170)	(2,374)	(1,598)
Other [Member]
Segment Reporting Information [Line Items]
Operating income (loss)									(1)		1
Other Income (Loss), Net										(1)	2
Gain (loss) on derivatives									(2)
Depreciation, depletion and amortization										1	1
Interest expense										1	1
Provision (benefit) for income taxes										(1)	1
Assets	353				264				353	264	271
Capital investments									49	25	55
Other [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues											(3)
Marketing [Member] | Midstream Services [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									$ (2,800)	$ (2,000)	$ (1,300)