CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 410,823	$ 400,534
Accounts receivable, net of allowance:
Trade	128,984	100,356
Oil and gas sales	304,255	344,552
Gas gathering, processing, and marketing	11,416	15,266
Oil and gas well equipment and supplies	53,375	49,722
Derivative instruments	72,943	15,151
Prepaid expenses	7,419	8,518
Other current assets	927	1,536
Total current assets	990,142	935,635
Oil and gas properties at cost, using the full cost method of accounting:
Proved properties	18,112,548	17,513,460
Unproved properties and properties under development, not being amortized	532,715	476,903
Gross oil and gas properties	18,645,263	17,990,363
Less—accumulated depreciation, depletion, amortization, and impairment	(15,000,443)	(14,748,833)
Net oil and gas properties	3,644,820	3,241,530
Fixed assets, net of accumulated depreciation of $312,927 and $290,114, respectively	238,964	210,922
Goodwill	620,232	620,232
Derivative instruments	2,330	2,086
Other assets	34,905	32,234
Total assets	5,531,393	5,042,639
Accounts payable:
Trade	74,596	68,883
Gas gathering, processing, and marketing	20,643	29,503
Accrued liabilities:
Exploration and development	146,886	115,762
Taxes other than income	24,392	23,687
Other	199,093	212,400
Derivative instruments	90,480	42,066
Revenue payable	180,869	187,273
Total current liabilities	736,959	679,574
Long-term debt:
Principal	1,500,000	1,500,000
Less—unamortized debt issuance costs and discount	(12,261)	(13,080)
Long-term debt, net	1,487,739	1,486,920
Deferred income taxes	201,350	101,618
Asset retirement obligation	159,568	158,421
Derivative instruments	11,511	4,268
Other liabilities	47,768	43,560
Total liabilities	2,644,895	2,474,361
Commitments and contingencies (Note 10)
Stockholders’ equity:
Preferred stock, $0.01 par value, 15,000,000 shares authorized, no shares issued	0	0
Common stock, $0.01 par value, 200,000,000 shares authorized, 95,392,547 and 95,437,434 shares issued, respectively	954	954
Additional paid-in capital	2,770,532	2,764,384
Retained earnings (accumulated deficit)	112,811	(199,259)
Accumulated other comprehensive income	2,201	2,199
Total stockholders’ equity	2,886,498	2,568,278
Total liabilities and stockholders' equity	$ 5,531,393	$ 5,042,639