CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands		3 Months Ended			6 Months Ended			12 Months Ended
		Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017		Dec. 31, 2017	Dec. 31, 2016
Operating Activities
Net income		$ 45,518	$ (17,571)	[1]	$ 213,771	$ 40,893	[1]
Adjustments to reconcile net income to net cash provided by continuing operations activities:
Loss (Income) from discontinued operations		398	217	[1]	835	(752)	[1]
Depreciation, depletion and amortization		237,997	234,992	[1]	468,730	471,146	[1]
Dry hole costs (credits)					(11)	1,904
Amortization of undeveloped leases					22,774	20,306
Accretion of asset retirement obligations		11,028	10,428	[1]	20,942	20,984	[1]
Deferred income tax (benefit) charge					(156,489)	33,130
Pretax (gain) loss from disposition of assets		(220)	1,333		118	(130,648)
Net decrease in noncash operating working capital					85,440	42,581
Other operating activities, net					(31,564)	91,918
Net cash provided by continuing operations activities					624,546	591,462
Investing Activities
Property additions and dry hole costs					(615,144)	(431,654)
Proceeds from sales of property, plant and equipment					623	64,303
Purchases of investment securities	[2]					(212,661)
Proceeds from maturity of investment securities	[2]					284,193
Net cash required by investing activities					(614,521)	(295,819)
Financing Activities
Capital lease obligation payments					(4,648)	(11,983)
Withholding tax on stock-based incentive awards					(6,922)	(7,081)
Cash dividends paid					(86,517)	(86,278)
Net cash provided by financing activities					(98,087)	(105,342)
Effect of exchange rate changes on cash and cash equivalents					24,382	(4,611)
Net increase (decrease) in cash and cash equivalents								$ (63,680)	$ 185,690
Cash and cash equivalents at beginning of period					964,988	872,797		872,797
Cash and cash equivalents at end of period		$ 901,308	$ 1,058,487		$ 901,308	$ 1,058,487		$ 964,988	$ 872,797

[1] Reclassified to conform to current presentation (see Notes A and B).

[2] Investments are Canadian government securities with maturities greater than 90 days at the date of acquisition.