Summary of Significant Accounting Policies - Cash flow statement as adjusted (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Initial Application Period [Line Items]
Other investing activities	$ 604	$ (2,767)
Net cash used in investing activities	(24,021)	(231,784)
Effect of foreign exchange rate changes on cash, cash equivalents, and restricted cash	(2,890)	4,806
Net decrease in cash, cash equivalents, and restricted cash		(29,801)
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	$ 421,449	363,218
As Previously Reported
Initial Application Period [Line Items]
Other investing activities		397
Net cash used in investing activities		(228,620)
Effect of foreign exchange rate changes on cash, cash equivalents, and restricted cash		4,781
Net decrease in cash, cash equivalents, and restricted cash		(26,662)
Cash, cash equivalents, and restricted cash at beginning of the period		389,275
Cash, cash equivalents, and restricted cash at end of the period		362,613
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		25
Net decrease in cash, cash equivalents, and restricted cash		(3,139)
Cash, cash equivalents, and restricted cash at beginning of the period		3,744
Cash, cash equivalents, and restricted cash at end of the period		$ 605