Commitments And Contingencies - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Cash and cash equivalents	$ 339,541	$ 482,339	$ 339,541	$ 482,339	$ 430,316	$ 450,193
Derivative, Amount of Hedged Item	200,000		200,000
Fair Value Hedge Liabilities	(9,405)		(9,405)		(2,990)
Foreign Currency Transaction Gain (Loss), before Tax	4,800		12,400
Other income (expense), net	(3,556)	$ (58)	(12,030)	$ (2,614)
Loans Payable to Bank	0		0		300,000
Notes Payable, Fair Value Disclosure	783,000		783,000
Cash repatriated			31,000
Angola, Kwanza
Loss Contingencies [Line Items]
Cash and cash equivalents	12,093		12,093		$ 26,909
Fair Value, Inputs, Level 2 [Member]
Loss Contingencies [Line Items]
Investments, Fair Value Disclosure	31,000		31,000
Available-for-sale Securities, Noncurrent	15,133		15,133
available-for-sale securities, noncurrent, mature in 2020	4,954		4,954
Available-for-sale securities, noncurrent, mature in 2023	10,179		10,179
Available-for-sales investments, amortized costs basis	$ 45,606		$ 45,606