Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Net income (loss)	$ (173,396)	$ 87,396
Net income (loss)	(173,396)	87,396
Adjustments to net income (loss) to reconcile to net cash provided by operating activities:
Net change in fair value of unsettled derivatives	120,920	(126,070)
Depreciation, depletion and amortization	262,412	235,329
Provision for uncollectible note receivable	0	(40,203)
Impairment of properties and equipment	192,742	29,759
Accretion of asset retirement obligations	2,573	3,434
Non-cash stock-based compensation	10,779	9,826
(Gain) loss on sale of properties and equipment	1,081	(692)
Amortization of debt discount and issuance costs	6,372	6,399
Deferred income taxes	(50,181)	50,767
Other	974	670
Changes in assets and liabilities	6,581	15,832
Net cash from operating activities	380,857	272,447
Cash flows from investing activities:
Capital expenditures for development of crude oil and natural gas properties	(432,635)	(334,406)
Capital expenditures for other properties and equipment	(2,450)	(2,299)
Acquisition of crude oil and natural gas properties, including settlement adjustments	(181,052)	5,372
Proceeds from sale of properties and equipment	1,782	1,293
Proceeds from divestiture	39,023	0
Proceeds from Sale of Notes Receivable	0	40,203
Restricted cash	1,249	(9,250)
Proceeds from Sale of Short-term Investments	0	49,890
Purchases of short-term investments	0	(49,890)
Net cash from investing activities	(574,083)	(299,087)
Cash flows from financing activities:
Proceeds from revolving credit facility	233,000	0
Repayment of revolving credit facility	(211,000)	0
Payments of Debt Issuance Costs	(4,060)	0
Purchase of treasury shares	4,494	5,274
Other	(719)	(645)
Net cash from financing activities	12,727	(5,919)
Net change in cash, cash equivalents, and restricted cash	(180,499)	(32,559)
Cash, cash equivalents and restricted cash, beginning of period	189,925	244,100
Cash, cash equivalents and restricted cash, end of period	9,426	211,541
Cash payments (receipts) for:
Interest, net of capitalized interest	27,817	32,647
Income taxes	393	(39)
Non-cash investing and financing activities:
Change in accounts payable related to purchases of properties and equipment	72,334	81,891
Change in asset retirement obligation, with a corresponding change to crude oil and natural gas properties, net of disposals	6,248	2,415
Purchase of properties and equipment under capital leases	$ 689	$ 2,160