Income Taxes - Reconciliation of Income Tax to Expense (Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income tax at federal statutory rate	$ (47,735)	$ (47,630)	$ (88,096)
State taxes, net of federal benefit	(5,803)	(4,316)	(65)
Non-deductible acquired IPR&D charge and other expenses	0	0	52,044
Change in valuation allowance	54,799	40,039	21,155
Impact of foreign rate change on deferred taxes	(12,405)	(2,950)	1,887
Other permanent differences	2,117	3,993	4,101
State tax rate benefit	164	(1,346)	(18)
Foreign rate differential	393	752	1,883
Stock-based compensation	2,484	1,180	(2,674)
Net operating losses surrendered under U.K.’s R&D tax relief scheme	0	0	9,349
State apportionment adjustments	482	(2,673)	48
Impact of foreign exchange rate differences	5,679	(4,532)	0
Credits and other	(70)	58	386
Income tax expense (benefit)	$ 105	$ (17,425)	$ 0