Income Taxes - Reconciliation (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes
Bermuda statutory income tax rate	0.00%	0.00%	0.00%
Foreign (U.S.) tax rate differential	(0.80%)	(0.50%)	(1.00%)
Research and development tax credits	2.40%	2.20%	1.50%
Share-based compensation	1.30%	(0.50%)	0.10%
Permanent differences	(0.10%)	(0.10%)
Change in valuation allowance	(5.70%)	(0.10%)
U.S. state taxes, net of federal	0.10%	(0.30%)	(0.40%)
FDII	0.60%	0.90%
Uncertain tax positions	(0.20%)	(0.30%)
Other	(0.90%)
Effective income tax rate	(3.30%)	1.30%	0.20%