Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 1,041	$ 974
Accounts receivable, net of allowance for doubtful accounts of $5,342 and $5,343 at June 30, 2013 and at December 31, 2012, respectively	156,706	146,046
Materials and supplies	9,314	8,563
Current derivative asset (Note 9)	9,895	16,552
Current income tax receivable	3,348	901
Current deferred tax asset	8,980	8,765
Prepaid expenses and other	10,385	13,843
Total current assets	199,669	195,644
Oil and natural gas properties on the full cost method:
Proved properties	4,015,975	3,822,381
Undeveloped leasehold not being amortized	543,535	521,659
Drilling equipment	1,487,769	1,478,645
Gas gathering and processing equipment	511,896	461,629
Transportation equipment	39,076	37,728
Other	70,065	62,840
Property, plant and equipment, gross, total	6,668,316	6,384,882
Less accumulated depreciation, depletion, amortization, and impairment	3,061,990	2,907,660
Net property and equipment	3,606,326	3,477,222
Debt issuance cost	12,638	13,432
Goodwill	62,808	62,808
Other intangible assets, net	171	680
Non-current derivative asset (Note 9)	4,887	0
Other assets	13,025	11,334
Total assets	3,899,524	3,761,120
Current liabilities:
Accounts payable	118,084	138,811
Accrued liabilities (Note 4)	58,832	54,098
Current portion of derivative liabilities (Note 9)	879	1,948
Current portion of other long-term liabilities (Note 5)	12,136	12,282
Total current liabilities	189,931	207,139
Long-term debt (Note 5)	715,474	716,359
Non-current derivative liabilities (Note 9)	0	562
Other long-term liabilities (Note 5)	160,907	166,983
Deferred income taxes	753,663	695,776
Shareholders' equity:
Preferred stock, $1.00 par value, 5,000,000 shares authorized, none issued	0	0
Common stock, $.20 par value, 175,000,000 shares authorized, 49,105,600 and 48,581,948 shares issued, respectively	9,643	9,594
Capital in excess of par value	435,467	423,603
Accumulated other comprehensive income (Note 11)	1,709	7,587
Retained earnings	1,632,730	1,533,517
Total shareholders' equity	2,079,549	1,974,301
Total liabilities and shareholders' equity	$ 3,899,524	$ 3,761,120