Income Taxes - Reconciliation of Statutory U.S. Federal Income Tax Rate to Company's Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
U.S. federal statutory rate	35.00%	35.00%	35.00%
State, net of federal benefit	(0.10%)	(0.50%)	0.00%
Stock-based compensation expense	(2.90%)	2.00%	0.50%
Tax exempt interest	(0.00%)	(0.20%)	(0.10%)
Research tax credit	3.20%	(1.20%)	(0.30%)
Foreign withholding tax	(25.20%)	24.70%	15.70%
Transaction costs	0.00%	2.40%	0.00%
Foreign tax rate differential	(20.80%)	0.40%	(2.80%)
Foreign tax credit	22.90%	(23.50%)	(15.30%)
Change in valuation allowance	(23.10%)	0.00%	(3.00%)
Re-measurement of deferred taxes	13.50%	0.00%	0.00%
Others	0.60%	(0.90%)	(0.40%)
Total	3.10%	38.20%	29.30%