Commitments And Contingencies - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Cash and cash equivalents	$ 367,150		$ 367,150		$ 430,316	$ 472,381	$ 450,193
Document Fiscal Year Focus			2018
Proceeds from Sale and Maturity of Available-for-sale Securities			$ 10,000
Proceeds from Sale of Available-for-sale Securities, Debt			62,021	$ 0
Derivative, Amount of Hedged Item	200,000		200,000
Fair Value Hedge Liabilities	(9,717)		(9,717)		(2,990)
Foreign Currency Transaction Gain (Loss), before Tax	(4,500)		(16,900)
Other income (expense), net	5,632	$ (1,287)	(6,398)	$ (3,901)
Loans Payable to Bank	0		0		300,000
Notes Payable, Fair Value Disclosure	790,000		790,000
Cash repatriated			57,000
Angola, Kwanza
Loss Contingencies [Line Items]
Cash and cash equivalents	13,786		13,786		$ 26,909
Fair Value, Inputs, Level 2 [Member]
Loss Contingencies [Line Items]
Proceeds from Sale of Available-for-sale Securities, Debt			51,900
Investments, Fair Value Disclosure	2,774		2,774
Available-for-sale Securities, Noncurrent	15,018		15,018
available-for-sale securities, noncurrent, mature in 2020	4,917		4,917
Available-for-sale securities, noncurrent, mature in 2023	10,101		10,101
Available-for-sales investments, amortized costs basis	$ 17,792		$ 17,792