CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands		3 Months Ended			12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017		Dec. 31, 2017
Operating Activities
Net income		$ 168,253	$ 58,464	[1]
Adjustments to reconcile net income to net cash provided by continuing operations activities:
(Income) loss from discontinued operations		437	(969)	[1]
Depreciation, depletion and amortization		230,733	236,154	[1]
Dry hole costs (credits)		(9)	2,904
Amortization of undeveloped leases		13,168	9,957
Accretion of asset retirement obligations		9,914	10,556	[1]
Deferred income tax (benefit) charge		(145,920)	58,533
Pretax loss (gain) from disposition of assets		339	(131,982)
Net decrease in noncash operating working capital		41,554	43,418
Other operating activities, net		(39,948)	18,478
Net cash provided by operating activities		278,521	305,513
Investing Activities
Property additions and dry hole costs		(273,901)	(211,631)
Proceeds from sales of property, plant and equipment		260	64,097
Purchases of investment securities	[2]		(212,661)
Proceeds from maturity of investment securities	[2]		113,210
Net cash required by investing activities		(273,641)	(246,985)
Financing Activities
Capital lease obligation payments		(2,404)	(9,660)
Withholding tax on stock-based incentive awards		(6,642)	(5,808)
Cash dividends paid		(43,258)	(43,136)
Net cash provided by financing activities		(52,304)	(58,604)
Effect of exchange rate changes on cash and cash equivalents		21,051	3,132
Net increase (decrease) in cash and cash equivalents		(26,373)	3,056
Cash and cash equivalents at beginning of period		964,988	872,797		$ 872,797
Cash and cash equivalents at end of period		$ 938,615	$ 875,853		$ 964,988

[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.