Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 701	$ 893
Accounts receivable (less allowance for doubtful accounts of $2,450 and $3,773 at December 31, 2017 and 2016, respectively)	111,512	83,954
Materials and supplies	505	3,340
Current derivative asset (Note 12)	721	0
Current deferred tax asset (Note 8)	0	25,211
Prepaid expenses and other	6,233	7,798
Total current assets	119,672	121,196
Oil and natural gas properties, on the full cost method:
Proved properties	5,712,813	5,446,305
Unproved properties not being amortized	296,764	314,867
Drilling equipment	1,593,611	1,565,268
Gas gathering and processing equipment	726,236	705,859
Saltwater disposal systems	62,618	60,638
Corporate land and building	59,080	59,066
Transportation equipment	29,631	32,842
Other	53,439	48,590
Property, plant and equipment, gross, total	8,534,192	8,233,435
Less accumulated depreciation, depletion, amortization, and impairment	6,151,450	5,952,330
Net property and equipment	2,382,742	2,281,105
Goodwill (Note 2)	62,808	62,808
Non-current derivative asset (Note 12)	0	377
Other assets	16,230	13,817
Total assets	2,581,452	2,479,303
Current liabilities:
Accounts payable	112,648	88,793
Accrued liabilities (Note 5)	48,523	39,651
Current derivative liabilities (Note 12)	7,763	21,564
Current portion of other long-term liabilities (Note 6)	13,002	14,907
Total current liabilities	181,936	164,915
Long-term debt less unamortized discount and debt issuance costs (Note 6)	820,276	800,917
Non-current derivative liabilities (Note 12)	0	415
Other long-term liabilities (Note 6)	100,203	103,064
Deferred income taxes (Note 8)	133,477	215,922
Commitments and contingencies (Note 14)	0	0
Shareholders' equity:
Preferred stock, $1.00 par value, 5,000,000 shares authorized, none issued	0	0
Common stock, $0.20 par value, 175,000,000 shares authorized, 52,880,134 and 51,494,318 shares issued as of December 31, 2017 and 2016, respectively	10,280	10,016
Capital in excess of par value	535,815	502,500
Accumulated other comprehensive income (net of tax of $39 at December 31, 2017) (Note 15)	63	0
Retained earnings	799,402	681,554
Total shareholders' equity	1,345,560	1,194,070
Total liabilities and shareholders' equity	$ 2,581,452	$ 2,479,303