Derivative Instruments (Schedule of Derivative Instruments in Statement of Financial Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Derivative assets, current	$ 10,858	$ 39,904
Derivative liabilities, current	(75,994)	(55,205)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	11,513	50,406
Derivative Asset, Fair Value, Gross Liability	(8,208)	(20,502)
Derivative, Fair Value, Net	3,305	29,904
Derivative assets, current	10,858	39,904
Derivative Asset, Gross Asset	19,066	60,406
Derivative Asset, Gross Liability	8,208	20,502
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	4,271	6,083
Derivative Asset, Fair Value, Gross Liability	(4,173)	(4,236)
Derivative, Fair Value, Net	98	1,847
Derivative Asset, Gross Asset	9,023	11,988
Derivative Asset, Gross Liability	4,173	4,236
Derivative assets, noncurrent	4,850	7,752
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(31,956)	(5,205)
Derivative Liability, Fair Value, Gross Liability	(30,843)	(15,345)
Derivative Liability, Fair Value, Gross Asset	(1,113)	10,140
Derivative liabilities, current	(75,994)	(55,205)
Derivative Deferred Premium, Net	0	0
Derivative Liability, Deferred Premiums, Gross Liability	(9,321)	(10,362)
Derivative Liability, Gross Liability	(84,202)	(75,707)
Derivative Liability, Deferred Premiums, Gross Asset	(9,321)	(10,362)
Derivative Liability, Gross Asset	8,208	20,502
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(13,182)	(10,233)
Derivative Liability, Fair Value, Gross Liability	(14,884)	(10,751)
Derivative Liability, Fair Value, Gross Asset	1,702	518
Derivative Deferred Premium, Net	0	0
Derivative Liability, Deferred Premiums, Gross Liability	(2,471)	(3,718)
Derivative Liability, Gross Liability	(32,900)	(45,053)
Derivative Liability, Deferred Premiums, Gross Asset	(2,471)	(3,718)
Derivative Liability, Gross Asset	4,173	4,236
Derivative liabilities, noncurrent	(28,727)	(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	1,803	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	218
Marcellus Shale Divestiture | Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Business Combination, Contingent Consideration, Asset, Noncurrent	670	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	3,926	5,000
Utica Shale Divestiture | Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Business Combination, Contingent Consideration, Asset, Noncurrent	2,279	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	(44,038)	(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	(15,545)	(30,584)
Derivative Liability, Contingent Payment, Gross Asset	0	0
Business Combination, Contingent Consideration, Liability, Noncurrent	$ 15,545	$ 30,584