Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes Disclosure [Line Items]
Statutory income tax rate			21.00%
Valuation allowance, increase (decrease) amount	$ (95,000,000.0)	$ (114,600,000)
Income tax research credits	$ 3,081,000	10,511,000	$ 17,295,000
Unrecognized tax benefits, period may increase or decrease due to tax examination	12 months
Unrecognized tax benefits, interest recognized	$ 0	0	0
Unrecognized tax benefits, penalties recognized	0	$ 0	$ 0
Federal
Income Taxes Disclosure [Line Items]
Net operating loss carryforward for income tax	2,000,000,000.0
Income tax research credits	98,900,000
Federal orphan drug credits	17,700,000
State
Income Taxes Disclosure [Line Items]
Net operating loss carryforward for income tax	1,300,000,000
Income tax research credits	$ 44,000,000.0