CONDENSED CONSOLIDATED BALANCE SHEET (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets
Cash and cash equivalents	$ 20,457	$ 30,736
Accounts receivable, net	172,661	172,419
Inventories	11,278	14,173
Deferred income taxes	29,095
Derivative instruments	4,215	50,824
Other current assets	1,174	2,158
Total current assets	238,880	270,310
Properties and equipment, net (Successful efforts method)	4,412,772	4,310,977
Other assets	36,184	35,026
TOTAL ASSETS	4,687,836	4,616,313
Current liabilities
Accounts payable	299,733	312,480
Current portion of long-term debt	75,000	75,000
Accrued liabilities	42,553	49,597
Income taxes payable	3,878	1,667
Deferred income taxes		5,203
Derivative instruments	26,100	192
Total current liabilities	447,264	444,139
Postretirement benefits	39,636	38,864
Long-term debt	1,052,000	1,012,000
Deferred income taxes	910,608	882,672
Asset retirement obligation	69,093	67,016
Derivative instruments	3,799
Other liabilities	42,222	40,175
Total liabilities	2,564,622	2,484,866
Commitments and contingencies
Stockholders' equity
Common stock: Authorized - 480,000,000 shares of $0.10 par value in 2013 and 2012, respectively. Issued-210,749,418 shares and 210,429,731 shares in 2013 and 2012, respectively	21,075	21,043
Additional paid-in capital	715,896	716,609
Retained earnings	1,411,887	1,373,264
Accumulated other comprehensive income/(loss)	(22,295)	23,880
Less treasury stock, at cost: 404,400 shares in 2013 and 2012, respectively	(3,349)	(3,349)
Total stockholders' equity	2,123,214	2,131,447
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 4,687,836	$ 4,616,313