Property and Equipment (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Oil and gas properties:
Proved properties	$ 730,977,000		$ 730,977,000		$ 682,276,000
Unproved properties	465,900,000		465,900,000		454,391,000
Support equipment and facilities	706,090,000		706,090,000		687,835,000
Total oil and gas properties	1,902,967,000		1,902,967,000		1,824,502,000
Less: accumulated depletion	(437,244,000)		(437,244,000)		(314,190,000)
Oil and gas properties, net	1,465,723,000		1,465,723,000		1,510,312,000
Other property	30,366,000		30,366,000		27,316,000
Less: accumulated depreciation	(14,125,000)		(14,125,000)		(11,866,000)
Other property, net	16,241,000		16,241,000		15,450,000
Property and equipment, net	1,481,964,000		1,481,964,000		1,525,762,000
Depletion expense	$ 56,400,000	$ 31,200,000	$ 113,100,000	$ 61,300,000