Asset Retirement Obligation (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Changes in the ARO
Asset retirement obligation — beginning of period	$ 226,525	$ 175,786
Liabilities incurred with properties acquired	18,756	50,487
Liabilities incurred with properties drilled	0	941
Liabilities settled during the period	(1,271)	(2,918)
Liabilities associated with properties sold	(4,572)	(5,891)
Current period accretion	5,201	8,120
Asset retirement obligation — end of period	$ 244,639	$ 226,525