Income Taxes - Effective Income Tax Reconciliation (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax at U.S. statutory rate	34.00%	34.00%	34.00%
State, net of federal benefit	8.30%	1.70%	3.70%
Foreign rate differential	(3.80%)	(0.30%)	(0.20%)
Share-based compensation	38.20%	(9.10%)	(7.00%)
Non-deductible expenses	(1.10%)	(0.20%)	(0.20%)
Tax credits	7.80%	(0.40%)	1.40%
Tax Cuts and Jobs Act impact	(220.20%)	0.00%	0.00%
Other	0.40%	(0.70%)	(1.20%)
Change in valuation allowance	126.60%	(29.20%)	(33.10%)
Total	(9.80%)	(4.20%)	(2.60%)