Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 8,061	$ 29,819
Accounts receivable, net	83,034	77,307
Inventories, net	15,662	10,510
Prepaid expenses	2,438	3,465
Derivative instruments	16,914	42,516
Total current assets	126,109	163,617
Property and equipment—at cost, net	64,687	65,601
Oil and natural gas properties, using the full cost method:
Proved	2,923,695	2,860,611
Unevaluated (excluded from the amortization base)	220,011	162,921
Accumulated depreciation, depletion, amortization and impairment	(1,331,137)	(1,290,356)
Total oil and natural gas properties	1,812,569	1,733,176
Derivative instruments	3,289	517
Assets held for sale	5,689	5,689
Other assets	38,003	38,952
Total assets	2,050,346	2,007,552
Liabilities and stockholders' equity
Accounts payable and accrued liabilities	123,769	101,598
Accrued payroll and benefits payable	15,453	19,655
Accrued interest payable	33,839	24,131
Revenue distribution payable	18,851	18,152
Current maturities of long-term debt	2,803	3,746
Derivative instruments	3,495	436
Deferred income taxes	13,607	26,872
Total current liabilities	211,817	194,590
Long-term debt, less current maturities	1,313,610	1,289,656
Derivative instruments	1,745	2,192
Stock-based compensation	3,003	3,042
Asset retirement obligations, net of current	46,586	46,314
Deferred income taxes	17,603	8,901
Commitments and contingencies (Note 10)
Stockholders' equity:
Preferred stock, 600,000 shares authorized, none issued and outstanding	0	0
Additional paid in capital	422,934	422,434
Retained earnings	15,996	17,186
Accumulated other comprehensive income, net of taxes	17,038	23,223
Total stockholders' equity	455,982	462,857
Total liabilities and stockholders' equity	2,050,346	2,007,552
Common Class A
Stockholders' equity:
Common stock	0	0
Common Class B
Stockholders' equity:
Common stock	4	4
Common Class C
Stockholders' equity:
Common stock	2	2
Common Class D
Stockholders' equity:
Common stock	3	3
Common Class E
Stockholders' equity:
Common stock	5	5
Common Class F
Stockholders' equity:
Common stock	0	0
Common Class G
Stockholders' equity:
Common stock	$ 0	$ 0