Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Contingency [Line Items]
Valuation allowance	$ (552,165,000)	$ (477,638,000)
Increase in valuation allowance	74,600,000	208,100,000	$ 194,400,000
Amount of unrecognized tax benefits that would impact the effective income tax rate	300,000
Amount of unrecognized tax benefits that would impact deferred tax assets	28,700,000
Income tax expense related to interest and penalties	0	0	$ 0
Her Majesty's Revenue and Customs (HMRC)
Income Tax Contingency [Line Items]
Net operating loss carryforwards	207,200,000	178,500,000
Federal
Income Tax Contingency [Line Items]
Net operating loss carryforwards	1,385,100,000	1,338,500,000
Federal | Research and development tax credit carryforward
Income Tax Contingency [Line Items]
Tax credit carryforwards	63,600,000	49,700,000
State
Income Tax Contingency [Line Items]
Net operating loss carryforwards	756,100,000	757,400,000
State | Research and development tax credit carryforward
Income Tax Contingency [Line Items]
Tax credit carryforwards	29,800,000	25,900,000
Foreign
Income Tax Contingency [Line Items]
Tax credit carryforwards	$ 1,800,000	$ 1,800,000