Derivative Instruments (Schedule of Derivative Instruments in Statement of Financial Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Derivative assets, current	$ 13,621	$ 39,904
Derivative liabilities, current	(64,751)	(55,205)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	21,704	50,406
Derivative Asset, Fair Value, Gross Liability	(15,582)	(20,502)
Derivative, Fair Value, Net	6,122	29,904
Derivative assets, current	13,621	39,904
Derivative Asset, Gross Asset	29,203	60,406
Derivative Asset, Gross Liability	(15,582)	(20,502)
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	10,303	6,083
Derivative Asset, Fair Value, Gross Liability	(8,243)	(4,236)
Derivative, Fair Value, Net	2,060	1,847
Derivative Asset, Gross Asset	14,258	11,988
Derivative Asset, Gross Liability	(8,243)	(4,236)
Derivative assets, noncurrent	6,015	7,752
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(18,366)	(5,205)
Derivative Liability, Fair Value, Gross Liability	(26,390)	(15,345)
Derivative Liability, Fair Value, Gross Asset	8,024	10,140
Derivative liabilities, current	(64,751)	(55,205)
Derivative Deferred Premium, Net	0	0
Derivative Liability, Deferred Premiums, Gross Liability	(7,558)	(10,362)
Derivative Liability, Gross Liability	(80,333)	(75,707)
Derivative Liability, Deferred Premiums, Gross Asset	7,558	10,362
Derivative Liability, Gross Asset	15,582	20,502
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(4,514)	(10,233)
Derivative Liability, Fair Value, Gross Liability	(10,988)	(10,751)
Derivative Liability, Fair Value, Gross Asset	6,474	518
Derivative Deferred Premium, Net	0	0
Derivative Liability, Deferred Premiums, Gross Liability	(1,769)	(3,718)
Derivative Liability, Gross Liability	(27,170)	(45,053)
Derivative Liability, Deferred Premiums, Gross Asset	1,769	3,718
Derivative Liability, Gross Asset	8,243	4,236
Derivative liabilities, noncurrent	(18,927)	(40,817)
Niobrara Divestiture | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Contingent Payment, Gross Liability	0	0
Contingent Consideration, Current Asset	3,409	5,000
Niobrara Divestiture | Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Business Combination, Contingent Consideration, Asset, Noncurrent	1,538	2,035
Derivative Asset, Contingent Payment, Gross Liability	0	0
Marcellus Shale Divestiture | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Contingent Payment, Gross Liability	0
Contingent Consideration, Current Asset	3
Marcellus Shale Divestiture | Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Business Combination, Contingent Consideration, Asset, Noncurrent	447	1,369
Derivative Asset, Contingent Payment, Gross Liability	0	0
Utica Shale Divestiture | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Contingent Payment, Gross Liability	0	0
Contingent Consideration, Current Asset	4,087	5,000
Utica Shale Divestiture | Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Business Combination, Contingent Consideration, Asset, Noncurrent	1,970	2,501
Derivative Asset, Contingent Payment, Gross Liability	0	0
ExL Acquisition | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Business Combination, Contingent Consideration, Liability, Current	(46,385)	(50,000)
Derivative Liability, Contingent Payment, Gross Asset	0	0
ExL Acquisition | Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Contingent Payment, Gross Liability	(14,413)	(30,584)
Derivative Liability, Contingent Payment, Gross Asset	0	0
Business Combination, Contingent Consideration, Liability, Noncurrent	$ (14,413)	$ (30,584)