Summary of Significant Accounting Policies - Cash flow statement as adjusted (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Initial Application Period [Line Items]
Other investing activities	$ 668	$ (2,940)
Net cash used in investing activities	54,003	(111,718)
Effect of foreign exchange rate changes on cash, cash equivalents, and restricted cash	(1,923)	3,640
Net decrease in cash, cash equivalents, and restricted cash		(34,230)
Cash, cash equivalents, and restricted cash at beginning of the period	450,125	393,019
Cash, cash equivalents, and restricted cash at end of the period	$ 548,581	358,789
As Previously Reported
Initial Application Period [Line Items]
Other investing activities		224
Net cash used in investing activities		(108,554)
Effect of foreign exchange rate changes on cash, cash equivalents, and restricted cash		3,613
Net decrease in cash, cash equivalents, and restricted cash		(31,093)
Cash, cash equivalents, and restricted cash at beginning of the period		389,275
Cash, cash equivalents, and restricted cash at end of the period		358,182
Accounting Standards Update 2016-18 | Adjustment
Initial Application Period [Line Items]
Other investing activities		(3,164)
Net cash used in investing activities		(3,164)
Effect of foreign exchange rate changes on cash, cash equivalents, and restricted cash		27
Net decrease in cash, cash equivalents, and restricted cash		(3,137)
Cash, cash equivalents, and restricted cash at beginning of the period		3,744
Cash, cash equivalents, and restricted cash at end of the period		$ 607