Sale and Exchange of Properties Sale and Exchange of Properties (Details) a in Thousands, $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2014 USD ($) a well	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net acres received | a	73
Number of producing wells received | well	900
Net acres transferred | a	138
Number of producing vertical wells transferred | well	2,000
Ownership interest transferred	50.00%
Amount of cash transferred	$ 167,300		$ 157,300	$ 0	$ 157,256
Nonconsolidated investment	50.00%		50.00%		50.00%
Fair value of assets exchanged	$ 516,500		$ 516,500		$ 516,500
Gain on sale / exchange of assets	34,100	$ 0	37,749	$ 0	37,749
Pre-tax gain related to the de-designation of derivative instruments	28,000
Acreage
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Fair value of assets exchanged	318,300		318,300		318,300
Wells
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Fair value of assets exchanged	$ 198,200		$ 198,200		$ 198,200