Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenues	$ 928.0		$ 868.0		$ 3,076.0		$ 2,464.0
Operating income (loss)	286		309		1,088		886
Other Gain, Net					1
Gain (loss) on derivatives	78		12		(29)		75
Depreciation, depletion and amortization	238		205		693		571
Interest expense	13	[1]	11	[1]	39	[1]	29	[1]
Provision (benefit) for income taxes	140	[1]	124	[1]	409	[1]	373	[1]
Assets	9,177.0		7,894.0		9,177.0		7,894.0		8,048.0
Capital investments	574	[2]	542	[2]	1,837	[2]	1,755	[2]
Change in accrued expenditures	53.0		15.0		114.0		26.0
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(710.0)		(602.0)		(2,450.0)		(1,744.0)
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenues	652.0		622.0		2,169.0		1,749.0
Operating income (loss)	189		223		817		651
Other Gain, Net					1
Gain (loss) on derivatives	79		11		(27)		74
Depreciation, depletion and amortization	223		192		650		534
Interest expense	10	[1]	9	[1]	29	[1]	21	[1]
Provision (benefit) for income taxes	107	[1]	88	[1]	309	[1]	282	[1]
Assets	7,461.0		6,265.0		7,461.0		6,265.0
Capital investments	531	[2]	496	[2]	1,706	[2]	1,603	[2]
Exploration and Production [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(3.0)		(1.0)		(13.0)		(4.0)
Exploration and Production [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Assets	78.0		65.0		78.0		65.0
Midstream Services [Member]
Segment Reporting Information [Line Items]
Revenues	276.0		246.0		907.0		715.0
Operating income (loss)	97		86		272		235
Gain (loss) on derivatives			1		(1)		1
Depreciation, depletion and amortization	15		13		43		37
Interest expense	2	[1]	2	[1]	9	[1]	8	[1]
Provision (benefit) for income taxes	34	[1]	36	[1]	101	[1]	91	[1]
Assets	1,494.0		1,388.0		1,494.0		1,388.0
Capital investments	34	[2]	40	[2]	109	[2]	135	[2]
Midstream Services [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(707.0)		(601.0)		(2,437.0)		(1,740.0)
Other [Member]
Segment Reporting Information [Line Items]
Operating income (loss)					(1)
Gain (loss) on derivatives	(1)				(1)
Interest expense	1	[1]			1	[1]
Provision (benefit) for income taxes	(1)	[1]			(1)	[1]
Assets	222.0	[3]	241.0	[3]	222.0	[3]	241.0	[3]
Capital investments	9	[2]	6	[2]	22	[2]	17	[2]
Marketing [Member] | Midstream Services [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	$ (612.0)		$ (515.0)		$ (2,161.0)		$ (1,494.0)

[1] Interest expense and the provision for income taxes by segment are allocated as they are incurred at the corporate level.

[2] Capital investments includes increases of $53 million and decreases of $15 million for the three month periods ended September 30, 2014 and 2013, respectively, and increases of $114 million and $26 million for the nine month periods ended September 30, 2014 and 2013, respectively, relating to the change in accrued expenditures between periods.

[3] Other assets represent corporate assets not allocated to segments and assets for non reportable segments.