Commitments And Contingencies - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Loss Contingencies [Line Items]
Notes Payable, Fair Value Disclosure	$ 489,000
Letters of Credit Outstanding, Amount	67,000	$ 54,000
Other Nonoperating Income (Expense)	(6,055)	(6,244)	$ (15,336)
Cash and Cash Equivalents, at Carrying Value	430,316	450,193	385,235	$ 430,714
Investments and Other Noncurrent Assets	68,280	59,130
Derivative, Amount of Hedged Item	200,000
Fair Value Hedge Liabilities	(2,990)	(1,557)
Other Income [Member]
Loss Contingencies [Line Items]
Other Nonoperating Income (Expense)	(100)	$ (7,300)	$ (17,400)
Angola, Kwanza
Loss Contingencies [Line Items]
Cash and Cash Equivalents, at Carrying Value	26,922
Angola Member
Loss Contingencies [Line Items]
Investments and Other Noncurrent Assets	69,986
Other Noncurrent Liabilities [Member]
Loss Contingencies [Line Items]
Fair Value Hedge Liabilities	(2,990)
Fair Value, Inputs, Level 2 [Member]
Loss Contingencies [Line Items]
Investments, Fair Value Disclosure	$ 68,041