Income Taxes - Reconciliation of Effective Tax Rate (Details)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Tax Disclosure [Abstract]
U.S. federal statutory tax rate	21.00%	21.00%
State income taxes, net of federal benefit	0.50%	(7.50%)
Non-U.S. income taxed at different rates	1.30%	20.70%
Reduction in tax benefit related to stock-based awards	(2.30%)	(44.20%)
Non-deductible expenses	(0.60%)	(8.30%)
Research and development credit	0.80%	24.70%
Increase in valuation allowance	(15.70%)	(3.10%)
Other	(0.20%)	0.00%
Effective income tax rate	4.80%	3.30%