Income Taxes - Summary (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Provision for income taxes
Provision for income taxes	$ 0	$ 0	$ 0
Net deferred tax assets, before valuation allowance	202,800,000	174,200,000	152,200,000
Unrecognized tax benefits	2,600,000	0
Research Tax Credit Carryforward
Provision for income taxes
Tax credit carryforward		26,100,000
Federal
Provision for income taxes
Net operating loss carryforwards	700,700,000	618,100,000	539,200,000
Operating loss carryforwards, subject to expiration	347,400,000
State
Provision for income taxes
Net operating loss carryforwards	$ 530,300,000	$ 527,100,000	$ 526,600,000