Commitments And Contingencies - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Cash and cash equivalents	$ 334,910	$ 462,516	$ 430,316	$ 450,193
Derivative, Amount of Hedged Item	200,000
Fair Value Hedge Liabilities	(6,353)		(2,990)
Other income (expense), net	(8,474)	(2,556)
Loans Payable to Bank	0		$ 300,000
Notes Payable, Fair Value Disclosure	791,000
Other Income [Member]
Loss Contingencies [Line Items]
Other income (expense), net		$ 500
Angola, Kwanza
Loss Contingencies [Line Items]
Cash and cash equivalents	21,904
Angola [Member]
Loss Contingencies [Line Items]
Investments and Other Noncurrent Assets	68,078
Fair Value, Inputs, Level 2 [Member]
Loss Contingencies [Line Items]
Investments, Fair Value Disclosure	$ 68,042