Property and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Oil and natural gas properties
Evaluated (subject to amortization)	$ 763,527	$ 423,945
Unproved and unevaluated (not subject to amortization)
Incurred in 2012	36,488
Incurred in 2011	24,138	60,934
Incurred in 2010	70,417	80,593
Incurred in 2009 and prior	18,632	21,071
Total unproved and unevaluated	149,675	162,598
Total oil and natural gas properties	913,202	586,543
Accumulated depletion	(344,609)	(201,543)
Net oil and natural gas properties	568,593	385,000
Other property and equipment
Other property and equipment	27,258	18,764
Accumulated depreciation	(4,761)	(3,899)
Net other property and equipment	22,497	14,865
Net property and equipment	591,090	399,865
Computer equipment [Member]
Other property and equipment
Other property and equipment	834	787
Furniture [Member]
Other property and equipment
Other property and equipment	793	458
Software [Member]
Other property and equipment
Other property and equipment	1,355	1,111
Other equipment [Member]
Other property and equipment
Other property and equipment	196	194
Leasehold improvements [Member]
Other property and equipment
Other property and equipment	644	627
Support equipment and facilities [Member]
Other property and equipment
Other property and equipment	$ 23,436	$ 15,587