Property and Equipment (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Oil and gas properties:
Proved properties	$ 702,352,000		$ 682,276,000
Unproved properties	501,699,000		454,391,000
Support equipment and facilities	696,569,000		687,835,000
Total oil and gas properties	1,900,620,000		1,824,502,000
Less: accumulated depletion	(373,353,000)		(314,190,000)
Oil and gas properties, net	1,527,267,000		1,510,312,000
Other property	28,143,000		27,316,000
Less: accumulated depreciation	(13,097,000)		(11,866,000)
Other property, net	15,046,000		15,450,000
Property and equipment, net	1,542,313,000		1,525,762,000
Depletion expense	56,600,000	30,100,000
Depletion costs included in crude oil inventory and other receivables	$ 2,500,000		$ 0