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Income Taxes - Reconciliation Between Statutory Federal Income Tax Rate and Effective Income Tax Rate (Detail)
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Income Taxes      
U.S. Federal statutory tax rate 21.00% 21.00% 21.00%
State taxes 5.90% 5.70% 5.70%
Research credit 0.90% 0.70% 0.80%
Orphan drug credit 6.60% 9.80% 8.80%
Other (0.30%) 0.40% (0.10%)
Change in valuation allowance (34.10%) (37.60%) (36.20%)