Fair Value Measurement - Rollforward of Unobservable Reconciliations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at December 31	$ 19,283
Change in fair value of contingent consideration	677	$ 10,417	$ (270)
Foreign exchange impact	(2,754)	(3,557)
Balance at December 31		19,283
Significant Unobservable Inputs (Level 3)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at December 31	19,283	22,162
Contingent consideration payments paid in stock	(15,345)	(11,374)
Change in fair value(1)	(4,678)	(2,012)
Change in fair value of contingent consideration		402
Foreign exchange impact	63	(312)
Balance at December 31	$ 0	$ 19,283	$ 22,162