CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities
Net Income (Loss)	$ 306,828	$ (167,513)	$ (945,731)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation, depletion and amortization	483,376	447,961	593,789
Asset impairment	1,671	220,652	1,292,308
Accretion of discount on asset retirement obligations	5,831	6,672	7,108
Deferred income taxes	(107,737)	(57,193)	(539,735)
Change in derivative fair value	17,143	76,490	233,315
Gain on sale of assets	(3,620)	(246,393)	(28,077)
Stock-based compensation expense	15,402	19,641	12,910
Exploration, including dry holes	2,130	16	7,097
Other, net	(10,488)	5,752	35,641
Net change in:
Accounts receivable	(85,465)	38,305	117,486
Inventories	1,045	(2,948)	(655)
Accounts payable	4,962	404	(41,560)
Accrued taxes/income tax receivable	(55,089)	(17,326)	(4,791)
Pension and other postretirement benefit contributions	(118)	(14,608)	(24,848)
Other current assets and liabilities	(6,508)	(14,855)	5,058
Net cash provided by operating activities	569,363	295,057	719,315
Investing Activities
Additions to oil and natural gas properties	(909,852)	(447,028)	(1,154,373)
Acquisitions	(276,840)	(147,879)	(87,410)
Proceeds from asset sales and sale of Alabama Gas Corporation	3,695	528,775	394,521
Purchase of short-term investments	0	0	(919,000)
Sale of short-term investments	0	0	919,000
Net cash used in investing activities	(1,182,997)	(66,132)	(847,262)
Financing Activities
Payment of dividends on common stock	0	0	(6,074)
Issuance of common stock, net	273	381,261	399,600
Taxes paid for shares withheld	(3,293)	(2,609)	(4,723)
Reduction of long-term debt	(24,000)	0	0
Net change in credit facility	255,000	(222,500)	(262,500)
Tax benefit (expense) on stock compensation	0	(256)	1,064
Net cash provided by financing activities	227,980	155,896	127,367
Net change in cash and cash equivalents	(385,654)	384,821	(580)
Cash and cash equivalents at beginning of period	386,093	1,272	1,852
Cash and cash equivalents at end of period	$ 439	$ 386,093	$ 1,272