Derivative Instruments (Schedule of Derivative Instruments in Statement of Financial Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Derivatives, Fair Value [Line Items]
Derivative assets, current	$ 10,928		$ 0
Derivative liabilities, current	(145,520)	$ (57,121)
Derivative liabilities, noncurrent	(87,933)	(112,332)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	32,422	4,869
Derivative Asset, Gross Asset	42,187	4,869
Derivative Asset, Fair Value, Gross Liability	(31,259)	(4,869)
Derivative Asset, Gross Liability	(31,259)	(4,869)
Derivative, Fair Value, Net	1,163	0
Derivative assets, current	10,928	0
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	13,418	9,505
Derivative Asset, Gross Asset	25,698	19,695
Derivative Asset, Fair Value, Gross Liability	(13,418)	(9,505)
Derivative Asset, Gross Liability	(13,418)	(9,505)
Derivative, Fair Value, Net	0	0
Derivative assets, noncurrent	12,280	10,190
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(97,140)	(57,121)
Derivative Deferred Premium, Net	0	0
Derivative Liability, Fair Value, Gross Liability	(118,953)	(52,671)
Derivative Liability, Deferred Premiums, Gross Liability	(9,446)	(9,319)
Derivative Liability, Contingent Payment, Gross Liability	(48,380)
Derivative Liability, Gross Liability	(176,779)	(61,990)
Derivative Liability, Fair Value, Gross Asset	21,813	(4,450)
Derivative Liability, Deferred Premiums, Gross Asset	9,446	(9,319)
Derivative Liability, Gross Asset	31,259	4,869
Business Combination, Contingent Consideration, Liability	(48,380)
Derivative liabilities, current	(145,520)	(57,121)
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(34,258)	(26,707)
Derivative Deferred Premium, Net	0	0
Derivative Liability, Fair Value, Gross Liability	(40,006)	(24,609)
Derivative Liability, Deferred Premiums, Gross Liability	(7,670)	(11,603)
Derivative Liability, Contingent Payment, Gross Liability	(53,675)	(85,625)
Derivative Liability, Gross Liability	(101,351)	(121,837)
Derivative Liability, Fair Value, Gross Asset	5,748	(2,098)
Derivative Liability, Deferred Premiums, Gross Asset	(7,670)	(11,603)
Derivative Liability, Contingent Payment, Gross Asset		0
Derivative Liability, Gross Asset	13,418	9,505
Business Combination, Contingent Consideration, Liability	(53,675)	(85,625)
Derivative liabilities, noncurrent	(87,933)	(112,332)
Niobrara Divestiture
Derivatives, Fair Value [Line Items]
Contingent Consideration, Current Asset	4,820
Contingent Consideration, Noncurrent Asset	5,150	0
Marcellus Shale Divestiture
Derivatives, Fair Value [Line Items]
Contingent Consideration, Current Asset	130
Contingent Consideration, Noncurrent Asset	1,400	2,205
Utica Shale Divestiture
Derivatives, Fair Value [Line Items]
Contingent Consideration, Current Asset	4,815
Contingent Consideration, Noncurrent Asset	$ 5,730	$ 7,985