Income Taxes - Reconciliation of Income Tax to Expense (Benefit) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax at federal statutory rate	$ (88,096,000)	$ (26,022,000)	$ (42,846,000)
State taxes, net of federal benefit	(65,000)	(8,000)	(19,000)
Non-deductible acquired IPR&D charge and other expenses	52,044,000	0	0
Change in valuation allowance	21,155,000	16,949,000	(11,208,000)
Impact of U.S. statutory tax rate change on revaluing deferred tax asset	0	0	36,085,000
Impact of foreign rate change on deferred tax assets	1,887,000	1,961,000	1,619,000
Other permanent differences	4,101,000	(701,000)	8,086,000
State tax rate benefit	(18,000)	169,000	56,000
Foreign rate differential	1,883,000	1,731,000	10,636,000
Stock-based compensation	(2,674,000)	(1,344,000)	(2,462,000)
Net operating losses surrendered under U.K.’s R&D tax relief scheme	9,349,000	6,322,000	0
Credits and other	434,000	943,000	53,000
Income tax benefit	$ 0	$ 0	$ 0