Property and Equipment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Property, Plant and Equipment
Subject to depletion	$ 728,839		$ 728,839		$ 576,497
Oil and Natural Gas Properties:
Not subject to depletion-acquisition costs	118,893		118,893		121,245
Gross oil and natural gas properties	847,732		847,732		697,742
Less accumulated depreciation, depletion, amortization and impairment	(170,286)		(170,286)		(145,102)
Net oil and natural gas properties capitalized	677,446		677,446		552,640
Other property and equipment	4,749		4,749		2,337
Less accumulated depreciation	(171,309)		(171,309)		(145,837)
Other property and equipment, net	3,726		3,726		1,602
Property and equipment, net of accumulated depreciation, depletion, amortization and impairment	681,172		681,172		554,242
Average depletetion rate per barrel equivalent unit of production	24.42	24.26	24.44	23.70
Capitalized general and administrative costs	843	1,083	1,640	1,775
Other property and equipment
Oil and Natural Gas Properties:
Less accumulated depreciation	(1,023)		(1,023)		(735)
Incurred in 2013
Oil and Natural Gas Properties:
Not subject to depletion-acquisition costs	5,984		5,984		0
Incurred in 2012
Oil and Natural Gas Properties:
Not subject to depletion-acquisition costs	109,059		109,059		117,395
Incurred in 2011
Oil and Natural Gas Properties:
Not subject to depletion-acquisition costs	1,670		1,670		1,670
Incurred in 2010
Oil and Natural Gas Properties:
Not subject to depletion-acquisition costs	1,647		1,647		1,647
Incurred in 2009
Oil and Natural Gas Properties:
Not subject to depletion-acquisition costs	$ 533		$ 533		$ 533
Minimum
Oil and Natural Gas Properties:
Number of years until unevaluated propertes are included in full cost pool			3 years
Maximum
Oil and Natural Gas Properties:
Number of years until unevaluated propertes are included in full cost pool			5 years