Income Taxes - Differences Between Federal and Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
U.S. federal statutory tax rate	21.00%	21.00%	34.00%
State and local taxes	4.80%	(1.50%)	2.40%
Research and development tax credit	3.10%	1.90%	3.00%
Stock-based compensation	19.00%	0.50%	0.00%
Uncertain tax positions	(0.50%)	(1.00%)	(0.20%)
Foreign tax rate differential	(7.90%)	(9.40%)	(4.00%)
Change in valuation allowance	(40.80%)	(12.60%)	2.70%
Gain on sale of intellectual property	(12.30%)	0.00%	0.00%
Transition tax	0.00%	0.00%	(2.70%)
Revaluation of U.S. deferred income taxes	0.00%	0.00%	(34.50%)
Other	(2.00%)	(2.20%)	(1.10%)
Effective tax rate	(15.60%)	(3.30%)	(0.40%)