CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands		3 Months Ended			9 Months Ended			12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017		Dec. 31, 2017	Dec. 31, 2016
Operating Activities
Net income (loss)		$ 93,944	$ (65,893)	[1]	$ 307,715	$ (25,000)	[1]
Adjustments to reconcile net income (loss) to net cash provided by continuing operations activities:
Loss (Income) from discontinued operations		1,815	(425)	[1]	2,650	(1,177)	[1]
Depreciation, depletion and amortization		241,833	243,636	[1]	710,563	714,782	[1]
Dry hole costs (credits)					4,526	(1,139)
Amortization of undeveloped leases					31,544	40,859
Accretion of asset retirement obligations		11,099	10,654	[1]	32,041	31,638	[1]
Deferred income tax benefit					(138,755)	(3,567)
Pretax gain from sale of assets		(124)	(117)		(6)	(130,765)
Net (increase) decrease in noncash operating working capital					(2,550)	1,070
Other operating activities, net					49,217	192,097
Net cash provided by continuing operations activities					996,945	818,798
Investing Activities
Property additions and dry hole costs					(858,356)	(706,417)
Proceeds from sales of property, plant and equipment					1,128	69,146
Purchases of investment securities	[2]					(212,661)
Proceeds from maturity of investment securities	[2]					320,828
Net cash required by investing activities					(857,228)	(529,104)
Financing Activities
Borrowings of debt, net of issuance costs						541,772
Repayments of debt						(550,000)
Capital lease obligation payments					(7,164)	(14,687)
Withholding tax on stock-based incentive awards					(6,922)	(7,151)
Cash dividends paid					(129,780)	(129,421)
Net cash required by financing activities					(143,866)	(159,487)
Effect of exchange rate changes on cash and cash equivalents					(13,107)	(5,797)
Net increase (decrease) in cash and cash equivalents								$ (17,256)	$ 124,410
Cash and cash equivalents at beginning of period					964,988	872,797		872,797
Cash and cash equivalents at end of period		$ 947,732	$ 997,207		$ 947,732	$ 997,207		$ 964,988	$ 872,797

[1] Reclassified to conform to current presentation (see Note B).

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