SUBSIDIARY GUARANTOR SUBSIDIARY GUARANTOR CASH FLOWS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Cash Provided by (Used in) Operating Activities [Abstract]
Cash flows from operating activities	$ 588,563	$ 486,263	$ 411,073
Net Cash Provided by (Used in) Investing Activities [Abstract]
Capital expenditures for development of crude oil and natural gas properties	(737,208)	(436,884)	(599,546)
Capital expenditures for other properties and equipment	(5,094)	(3,464)	(5,122)
Acquisition of crude oil and natural gas properties, including settlement adjustments and deposit for pending acquisition	(15,628)	(1,073,723)	0
Proceeds from sale of properties and equipment	9,991	4,945	405
Sale of promissory note	40,203	0	0
Restricted cash	(9,250)	0	0
Sale of short-term investments	49,890	0	0
Purchase of short-term investments	(49,890)	0	0
Intercompany transfers	0	0
Net cash from investing activities	(716,986)	(1,509,126)	(604,263)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from issuance of equity, net of issuance costs	0	855,074	202,851
Proceeds from issuance of senior notes	592,366	392,172	0
Proceeds from issuance of convertible senior notes	0	193,935	0
Proceeds from revolving credit facility	0	85,000	397,000
Repayment of revolving credit facility	0	(122,000)	(416,000)
Redemption of convertible notes	0	(115,000)	0
Redemption of senior notes	(519,375)	0	0
Payment of debt issuance costs	(50)	(15,556)	(974)
Purchase of treasury shares	(6,672)	(6,935)	(6,055)
Other	(1,271)	(577)	1,152
Intercompany transfers	0	0
Net cash from financing activities	64,998	1,266,113	177,974
Net change in cash and cash equivalents	(63,425)	243,250	(15,216)
Cash and cash equivalents, beginning of year	244,100	850	16,066
Cash and cash equivalents, end of year	180,675	244,100	850
Parent [Member]
Net Cash Provided by (Used in) Operating Activities [Abstract]
Cash flows from operating activities	537,704	492,893
Net Cash Provided by (Used in) Investing Activities [Abstract]
Capital expenditures for development of crude oil and natural gas properties	(439,897)	(436,361)
Capital expenditures for other properties and equipment	(3,539)	(2,282)
Acquisition of crude oil and natural gas properties, including settlement adjustments and deposit for pending acquisition	(21,000)	(1,076,256)
Proceeds from sale of properties and equipment	10,084	4,945
Sale of promissory note	40,203
Restricted cash	(9,250)
Sale of short-term investments	49,890
Purchase of short-term investments	(49,890)
Intercompany transfers	(239,191)	(9,415)
Net cash from investing activities	(662,590)	(1,519,369)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from issuance of equity, net of issuance costs		855,074
Proceeds from issuance of senior notes	592,366	392,172
Proceeds from issuance of convertible senior notes		193,935
Proceeds from revolving credit facility		85,000
Repayment of revolving credit facility		(122,000)
Redemption of convertible notes		(115,000)
Redemption of senior notes	(519,375)
Payment of debt issuance costs	(50)	(15,556)
Purchase of treasury shares	(6,672)	(6,935)
Other	(1,195)	(577)
Intercompany transfers	0	0
Net cash from financing activities	65,074	1,266,113
Net change in cash and cash equivalents	(59,812)	239,637
Cash and cash equivalents, beginning of year	240,487	850
Cash and cash equivalents, end of year	180,675	240,487	850
Guarantor Subsidiaries [Member]
Net Cash Provided by (Used in) Operating Activities [Abstract]
Cash flows from operating activities	50,859	(6,630)
Net Cash Provided by (Used in) Investing Activities [Abstract]
Capital expenditures for development of crude oil and natural gas properties	(297,311)	(523)
Capital expenditures for other properties and equipment	(1,555)	(1,182)
Acquisition of crude oil and natural gas properties, including settlement adjustments and deposit for pending acquisition	5,372	2,533
Proceeds from sale of properties and equipment	(93)	0
Sale of promissory note	0
Restricted cash	0
Sale of short-term investments	0
Purchase of short-term investments	0
Intercompany transfers	0	0
Net cash from investing activities	(293,587)	828
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from issuance of equity, net of issuance costs		0
Proceeds from issuance of senior notes	0	0
Proceeds from issuance of convertible senior notes		0
Proceeds from revolving credit facility		0
Repayment of revolving credit facility		0
Redemption of convertible notes		0
Redemption of senior notes	0
Payment of debt issuance costs	0	0
Purchase of treasury shares	0	0
Other	(76)	0
Intercompany transfers	239,191	9,415
Net cash from financing activities	239,115	9,415
Net change in cash and cash equivalents	(3,613)	3,613
Cash and cash equivalents, beginning of year	3,613	0
Cash and cash equivalents, end of year	0	3,613	0
Consolidation, Eliminations [Member]
Net Cash Provided by (Used in) Operating Activities [Abstract]
Cash flows from operating activities	0	0
Net Cash Provided by (Used in) Investing Activities [Abstract]
Capital expenditures for development of crude oil and natural gas properties	0	0
Capital expenditures for other properties and equipment	0	0
Acquisition of crude oil and natural gas properties, including settlement adjustments and deposit for pending acquisition	0	0
Proceeds from sale of properties and equipment	0	0
Sale of promissory note	0
Restricted cash	0
Sale of short-term investments	0
Purchase of short-term investments	0
Intercompany transfers	239,191	9,415
Net cash from investing activities	239,191	9,415
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from issuance of equity, net of issuance costs		0
Proceeds from issuance of senior notes	0	0
Proceeds from issuance of convertible senior notes		0
Proceeds from revolving credit facility		0
Repayment of revolving credit facility		0
Redemption of convertible notes		0
Redemption of senior notes	0
Payment of debt issuance costs	0	0
Purchase of treasury shares	0	0
Other	0	0
Intercompany transfers	(239,191)	(9,415)
Net cash from financing activities	(239,191)	(9,415)
Net change in cash and cash equivalents	0	0
Cash and cash equivalents, beginning of year	0	0
Cash and cash equivalents, end of year	$ 0	$ 0	$ 0