CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Operating Activities
Net income (loss) including noncontrolling interest		$ 419,486,000	$ (311,789,000)	[1]	$ (275,970,000)	[1]
Adjustments to reconcile net loss to net cash provided by continuing operations activities:
Loss from discontinued operations		3,522,000	853,000	[1]	2,027,000	[1]
Depreciation, depletion and amortization		971,901,000	957,719,000	[1]	1,054,081,000	[1]
Impairment of assets		20,000,000	0		95,088,000	[1]
Amortization of deferred major repair costs					3,794,000
Dry hole costs (credits)		20,624,000	(4,163,000)		15,047,000
Amortization of undeveloped leases		40,177,000	61,776,000		43,417,000
Accretion of asset retirement obligations		44,559,000	42,590,000	[1]	46,742,000	[1]
Deferred income tax expense (benefit)		(183,680,000)	260,420,000		(387,843,000)
Pretax gains from disposition of assets		(54,000)	(127,434,000)		(1,663,000)
Net (increase) decrease in noncash operating working capital		(169,808,000)	136,414,000		(38,689,000)
Other operating activities, net		52,669,000	111,689,000		44,764,000
Net cash provided by continuing operations activities		1,219,396,000	1,128,075,000		600,795,000
Investing Activities
Acquisition of oil properties		(794,623,000)
Property additions and dry hole costs		(1,102,805,000)	(1,009,667,000)		(926,948,000)
Proceeds from sales of property, plant and equipment		1,383,000	69,506,000		1,155,144,000
Purchase of investment securities	[2]		(212,661,000)		(695,879,000)
Proceeds from maturity of investment securities	[2]		320,828,000		761,000,000
Other investing activities, net					(7,230,000)
Net cash provided (required) by investing activities		(1,896,045,000)	(831,994,000)		286,087,000
Financing Activities
Borrowings of debt		325,000,000	541,597,000		541,444,000
Repayments of debt			(550,000,000)		(600,000,000)
Capital lease obligation payments		(9,750,000)	(17,133,000)		(10,447,000)
Issue cost of debt facility		(6,366,000)			(14,085,000)
Cash dividends paid		(173,044,000)	(172,565,000)		(206,635,000)
Other financing activities, net		(8,076,000)	(7,116,000)		(1,158,000)
Net cash required by financing activities		127,764,000	(205,217,000)		(290,881,000)
Effect of exchange rate changes on cash and cash equivalents		(28,730,000)	1,327,000		(6,387,000)
Net increase (decrease) in cash and cash equivalents		(577,615,000)	92,191,000		589,614,000
Cash and cash equivalents at beginning of period		964,988,000	872,797,000		283,183,000
Cash and cash equivalents at end of period		$ 387,373,000	$ 964,988,000		$ 872,797,000

[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.