Income Taxes - Reconciliation of Income Tax to Expense (Benefit) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income tax at federal statutory rate	$ (47,630,000)	$ (88,096,000)	$ (26,022,000)
State taxes, net of federal benefit	(4,316,000)	(65,000)	(8,000)
Non-deductible acquired IPR&D charge and other expenses	0	52,044,000	0
Change in valuation allowance	40,039,000	21,155,000	16,949,000
Impact of foreign rate change on deferred taxes	(2,950,000)	1,887,000	1,961,000
Other permanent differences	3,993,000	4,101,000	(701,000)
State tax rate benefit	(1,346,000)	(18,000)	169,000
Foreign rate differential	752,000	1,883,000	1,731,000
Stock-based compensation	1,180,000	(2,674,000)	(1,344,000)
Net operating losses surrendered under U.K.’s R&D tax relief scheme	0	9,349,000	6,322,000
State apportionment adjustments	(2,673,000)	48,000	0
Impact of foreign exchange rate differences	(4,532,000)	0	0
Credits and other	58,000	386,000	943,000
Income tax benefit	$ (17,425,000)	$ 0	$ 0