Segment Information (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues		$ 737.0	$ 651.0				$ 2,394.0	$ 1,752.0
Depreciation, depletion and amortization expense		135.0	99.0				364.0	349.0
Impairment of natural gas and oil properties			817.0	$ 470.0	$ 1,034.0			2,321.0
Operating income (loss)		110.0	(725.0)				564.0	(2,317.0)
Interest expense		31.0	26.0				97.0	57.0
Gain (loss) on derivatives		45.0	71.0				295.0	(28.0)
Loss on Early Extinguishment of Debt	$ 59.0	(59.0)	(51.0)			$ (11.0)	(70.0)	(51.0)
Other income (loss), net		(2.0)	3.0				6.0
Benefit for income taxes		(14.0)	(20.0)				(14.0)	(20.0)
Assets	7,202.0	7,202.0	6,890.0				7,202.0	6,890.0	$ 7,076.0
Capital investments		331.0	180.0				946.0	376.0
Restructuring charges			2.0					77.0
Cash and cash equivalents	989.0	989.0	1,474.0				989.0	1,474.0	$ 1,423.0	$ 15.0
Change in accrued expenditures		(2.0)	27.0				(13.0)	(24.0)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues		(467.0)	(409.0)				(1,579.0)	(1,108.0)
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenues		475.0	381.0				1,572.0	1,015.0
Depreciation, depletion and amortization expense		120.0	83.0				317.0	300.0
Impairment of natural gas and oil properties			817.0					2,321.0
Operating income (loss)		64.0	(777.0)				435.0	(2,486.0)
Interest expense		31.0	26.0				97.0	56.0
Gain (loss) on derivatives		45.0	71.0				295.0	(27.0)
Other income (loss), net		1.0	2.0				5.0	3.0
Benefit for income taxes		(14.0)	(20.0)				(14.0)	(20.0)
Assets	4,842.0	4,842.0	4,015.0				4,842.0	4,015.0
Capital investments		320.0	179.0				921.0	372.0
Restructuring charges			2.0					74.0
Exploration and Production [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues		5.0	3.0				13.0	17.0
Midstream Services [Member]
Segment Reporting Information [Line Items]
Revenues		262.0	270.0				822.0	737.0
Depreciation, depletion and amortization expense		15.0	16.0				47.0	49.0
Operating income (loss)		46.0	52.0				129.0	169.0
Interest expense								1.0
Gain (loss) on derivatives								(1.0)
Other income (loss), net		(3.0)	1.0				1.0	(2.0)
Assets	1,240.0	1,240.0	1,253.0				1,240.0	1,253.0
Capital investments		9.0	1.0				21.0	3.0
Restructuring charges								3.0
Midstream Services [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues		(472.0)	(412.0)				(1,592.0)	(1,125.0)
Other [Member]
Segment Reporting Information [Line Items]
Loss on Early Extinguishment of Debt		(59.0)	(51.0)				(70.0)	(51.0)
Other income (loss), net								(1.0)
Assets	1,120.0	1,120.0	1,622.0				1,120.0	1,622.0
Capital investments		2.0					4.0	1.0
Cash and cash equivalents	$ 989.0	989.0	1,500.0				989.0	1,500.0
Marketing [Member] | Midstream Services [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues		$ (422.0)	$ (355.0)				$ (1,436.0)	$ (941.0)