Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Line Items]
Income tax expense	$ 0	$ 0
Increase in valuation allowance due to deferred tax assets	47,200,000	40,200,000
Uncertain tax positions	0
Income tax expense, interest or penalties	$ 0	0
Orphan Drug
Income Tax Disclosure [Line Items]
Tax credit carryforwards		29,000,000.0
Tax credit carryforwards expiration year	2040
Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 265,100,000
Net operating loss carryforwards indefinitely		$ 233,600,000
Federal | Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforwards	34,200,000
State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	271,500,000
State | Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 7,200,000
Federal and State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards expiration year	2040
Federal and State | Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforwards expiration year	2040