Condensed Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 8,914	$ 312
Restricted short-term investments	125	125
Accounts receivable	138,649	122,159
Deferred income taxes	185	703
Derivative instruments	18,377	14,661
Prepaid expenses and other	19,005	19,065
Total current assets	185,255	157,025
Oil and natural gas properties (successful efforts basis), buildings and equipment, net	3,240,447	3,128,502
Derivative instruments	34,867	10,891
Other assets	25,933	28,984
TOTAL ASSETS	3,486,502	3,325,402
Current liabilities:
Accounts payable	105,912	175,893
Revenue and royalties payable	42,618	57,021
Accrued liabilities	46,166	51,151
Derivative instruments	0	1,111
Deferred income taxes	6,598	1,456
Total current liabilities	404,998	286,632
Long-term liabilities:
Deferred income taxes	311,449	255,471
Senior secured revolving credit facility	646,000	562,900
Asset retirement obligations, noncurrent	94,424	82,316
Derivative instruments	0	1,239
Other long-term liabilities	23,127	19,136
Total long-term liabilities	1,975,000	2,023,979
Shareholders' equity:
Preferred stock, $0.01 par value, 2,000,000 shares authorized; no shares outstanding	0	0
Capital in excess of par value	371,425	364,710
Retained earnings	734,534	649,539
Total shareholders' equity	1,106,504	1,014,791
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	3,486,502	3,325,402
Class A
Capital stock, $0.01 par value:
Common Stock	527	524
Class B
Capital stock, $0.01 par value:
Common Stock	18	18
10.25% Senior notes due 2014, net of unamortized discount
Current liabilities:
Senior Notes, Current	203,704	0
Long-term liabilities:
Senior notes	0	202,917
6.75% Senior notes due 2020
Long-term liabilities:
Senior notes	300,000	300,000
6.375% Senior notes due 2022
Long-term liabilities:
Senior notes	$ 600,000	$ 600,000