Income Taxes (Effective Income Tax Rates) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Income Tax Disclosure [Line Items]
Income tax expense (benefit) based on the U.S. statutory tax rate	$ 25,131	$ (172,590)		$ (1,148,792)
Revaluation of deferred tax (U.S. tax reform)	118,004
Deferred tax effect of deemed repatriation of foreign invested earnings (U.S. tax reform)	156,000
Deferred tax effect on Canadian earnings no longer indefinitely invested	65,000
Foreign income (loss) subject to foreign tax rates different than the U.S. statutory rate	12,658	8,582		49,739
State income taxes, net of federal benefit	2,438	9,090		(3,441)
U.S. tax benefit on certain foreign upstream investments	(32,926)	(21,336)		(16,939)
Deferred tax on distribution of foreign earnings				188,461
Increase in deferred tax asset valuation allowance related to other foreign exploration expenditures	18,601	25,734		40,788
Other, net	17,832	18,741		(13,747)
Total income tax expense (benefit)	$ 382,738	(219,172)	[1]	(1,026,490)	[1]
Canada [Member]
Income Tax Disclosure [Line Items]
Tax effects on sale of assets		(89,473)
Malaysia [Member]
Income Tax Disclosure [Line Items]
Tax effects on sale of assets		$ 2,080		$ (122,559)

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