Property and Equipment (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Oil and gas properties:
Proved properties	$ 1,177,379,000		$ 1,156,868,000
Unproved properties	366,717,000		363,717,000
Support equipment and facilities	1,017,103,000		968,722,000
Total oil and gas properties	2,561,199,000		2,489,307,000
Less: accumulated depletion	(750,778,000)		(716,121,000)
Oil and gas properties, net	1,810,421,000		1,773,186,000
Other property	33,950,000		33,718,000
Less: accumulated depreciation	(23,217,000)		(22,058,000)
Other property, net	10,733,000		11,660,000
Property and equipment, net	1,821,154,000		1,784,846,000
Depletion expense	$ 34,700,000	$ 44,000,000