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