Subsidiary Guarantor Condensed Consolidating Statement of Cash Flows (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Net cash from operating activities	$ 411,402,000	$ 360,831,000
Cash flows from investing activities:
Capital expenditures for development of crude oil and natural gas properties	(528,850,000)	(352,213,000)
Capital expenditures for other properties and equipment	(3,740,000)	(1,509,000)
Acquisition of crude oil and natural gas properties, including final settlement adjustments	(14,482,000)
Proceeds from sale of properties and equipment	3,322,000	4,945,000
Sale of promissory note	40,203,000	0
Restricted cash	(9,250,000)	0
Sale of short-term investments	49,890,000	0
Purchases of short-term investments	(49,890,000)	0
intercompany Transfer Investing Activities	0
Net cash from investing activities	(512,797,000)	(448,777,000)
Cash flows from financing activities:
Purchase of treasury shares	(5,325,000)	(5,106,000)
Other	(951,000)	593,000
Intercompany Transfers Financing Activities	0
Net cash from financing activities	(6,276,000)	1,284,788,000
Net change in cash and cash equivalents	(107,671,000)	1,196,842,000
Cash and cash equivalents, beginning of period	244,100,000	850,000
Cash and cash equivalents, end of period	136,429,000	$ 1,197,692,000
Consolidation, Eliminations [Member]
Net cash from operating activities	0
Cash flows from investing activities:
Capital expenditures for development of crude oil and natural gas properties	0
Capital expenditures for other properties and equipment	0
Acquisition of crude oil and natural gas properties, including final settlement adjustments	0
Proceeds from sale of properties and equipment	0
Sale of promissory note	0
Restricted cash	0
Sale of short-term investments	0
Purchases of short-term investments	0
intercompany Transfer Investing Activities	189,239,000
Net cash from investing activities	189,239,000
Cash flows from financing activities:
Purchase of treasury shares	0
Other	0
Intercompany Transfers Financing Activities	(189,239,000)
Net cash from financing activities	(189,239,000)
Net change in cash and cash equivalents	0
Cash and cash equivalents, beginning of period	0
Cash and cash equivalents, end of period	0
Parent Company [Member] | Reportable Legal Entities [Member]
Net cash from operating activities	382,715,000
Cash flows from investing activities:
Capital expenditures for development of crude oil and natural gas properties	(315,718,000)
Capital expenditures for other properties and equipment	(2,488,000)
Acquisition of crude oil and natural gas properties, including final settlement adjustments	(19,761,000)
Proceeds from sale of properties and equipment	3,322,000
Sale of promissory note	40,203,000
Restricted cash	(9,250,000)
Sale of short-term investments	49,890,000
Purchases of short-term investments	(49,890,000)
intercompany Transfer Investing Activities	(189,239,000)
Net cash from investing activities	(492,931,000)
Cash flows from financing activities:
Purchase of treasury shares	(5,325,000)
Other	(906,000)
Intercompany Transfers Financing Activities	0
Net cash from financing activities	(6,231,000)
Net change in cash and cash equivalents	(116,447,000)
Cash and cash equivalents, beginning of period	240,487,000
Cash and cash equivalents, end of period	124,040,000
Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Net cash from operating activities	28,687,000
Cash flows from investing activities:
Capital expenditures for development of crude oil and natural gas properties	(213,132,000)
Capital expenditures for other properties and equipment	(1,252,000)
Acquisition of crude oil and natural gas properties, including final settlement adjustments	5,279,000
Proceeds from sale of properties and equipment	0
Sale of promissory note	0
Restricted cash	0
Sale of short-term investments	0
Purchases of short-term investments	0
intercompany Transfer Investing Activities	0
Net cash from investing activities	(209,105,000)
Cash flows from financing activities:
Purchase of treasury shares	0
Other	(45,000)
Intercompany Transfers Financing Activities	189,239,000
Net cash from financing activities	189,194,000
Net change in cash and cash equivalents	8,776,000
Cash and cash equivalents, beginning of period	3,613,000
Cash and cash equivalents, end of period	$ 12,389,000