CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Operating Activities
Net loss		$ (311,789,000)	$ (275,970,000)	[1]	$ (2,270,833,000)	[1]
Adjustments to reconcile net loss to net cash provided by continuing operations activities:
Loss from discontinued operations		853,000	2,027,000	[1]	15,061,000	[1]
Depreciation, depletion and amortization		957,719,000	1,054,081,000	[1]	1,619,824,000	[1]
Impairment of assets		0	95,088,000	[1]	2,493,156,000	[1]
Amortization of deferred major repair costs			3,794,000		7,296,000
Dry hole costs (credits)		(4,163,000)	15,047,000		296,845,000
Amortization of undeveloped leases		61,776,000	43,417,000		75,312,000
Accretion of asset retirement obligations		42,590,000	46,742,000	[1]	48,665,000	[1]
Deferred income tax expense (benefit)		260,420,000	(387,843,000)		(978,030,000)
Pretax gains from disposition of assets		(127,434,000)	(1,663,000)	[1]	(154,155,000)	[1]
Net (increase) decrease in noncash operating working capital		136,414,000	(38,689,000)		35,064,000
Other operating activities, net		113,289,000	44,764,000		(4,836,000)
Net cash provided by continuing operations activities		1,129,675,000	600,795,000		1,183,369,000
Investing Activities
Property additions and dry hole costs		(1,009,667,000)	(926,948,000)		(2,549,736,000)
Proceeds from sales of property, plant and equipment		69,506,000	1,155,144,000		423,911,000
Purchases of investment securities	[2]	(212,661,000)	(695,879,000)		(911,787,000)
Proceeds from maturity of investment securities	[2]	320,828,000	761,000,000		1,129,139,000
Other investing activities, net			(7,230,000)		(13,648,000)
Net cash provided (required) by investing activities		(831,994,000)	286,087,000		(1,922,121,000)
Financing Activities
Borrowings of debt		541,597,000	541,444,000		600,000,000
Repayments of debt		(550,000,000)	(600,000,000)		(450,000,000)
Capital lease obligation payments		(17,133,000)	(10,447,000)		(10,434,000)
Purchase of treasury stock					(250,000,000)
Issue cost of debt facility			(14,085,000)
Cash dividends paid		(172,565,000)	(206,635,000)		(244,998,000)
Other financing activities, net		(7,116,000)	(1,158,000)		(9,129,000)
Net cash required by financing activities		(205,217,000)	(290,881,000)		(364,561,000)
Cash Flows from Discontinued Operations
Operating activities		10,905,000			(15,005,000)
Investing activities					5,314,000
Changes in cash included in current assets held for sale		(12,505,000)			192,585,000
Net increase (decrease) in cash and cash equivalents of discontinued operations		(1,600,000)			182,894,000
Effect of exchange rate changes on cash and cash equivalents		1,327,000	(6,387,000)		10,294,000
Net increase (decrease) in cash and cash equivalents		92,191,000	589,614,000		(910,125,000)
Cash and cash equivalents at beginning of period		872,797,000	283,183,000		1,193,308,000
Cash and cash equivalents at end of period		$ 964,988,000	$ 872,797,000		$ 283,183,000

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

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