Income Taxes - Provision Difference from Federal Statutory Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective tax rate of provision for income taxes difference from federal statutory rate
Federal statutory income tax rate (as a percent)		34.00%	34.00%	34.00%
State taxes, net of federal benefit (as a percent)		0.50%	6.50%	(2.70%)
Warrant revaluation (as a percent)			(4.30%)	(0.20%)
Research credit (as a percent)		2.60%	1.80%	1.10%
Foreign tax rate difference (as a percent)		(1.20%)	(1.60%)	(11.80%)
Change in tax law (as a percent)		(31.20%)
Change in valuation allowance (as a percent)		(5.20%)	(36.00%)	(19.90%)
Other (as a percent)		0.50%	(0.40%)	(0.50%)
Tax expense from decrease in net deferred tax assets for corporate tax rate reduction under Tax Act		$ 11.5
Corresponding adjustment to valuation allowance for corporate tax rate reduction under Tax Act		$ 11.5
State deferred tax effect on federal deferred tax (as a percent)		79.00%	66.00%
Provisional transition tax		$ 0.0
Maximum
Effective tax rate of provision for income taxes difference from federal statutory rate
Federal statutory income tax rate (as a percent)		35.00%
Forecast
Effective tax rate of provision for income taxes difference from federal statutory rate
Federal statutory income tax rate (as a percent)	21.00%