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Income Taxes - Reconciliation Between Statutory Tax Rate and Effective Tax Rate (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Income Tax Disclosure [Abstract]      
Federal tax at statutory rate 21.00% 21.00% 21.00%
State taxes 2.60% 1.80% 1.70%
Change in valuation allowance (3.50%) (357.60%) (16.40%)
Research and development credit (5.40%) (10.90%) (9.10%)
Orphan drug credit (1.30%) 17.10% (2.70%)
Section 162(m) limitation 1.70% 2.70% 3.10%
Other tax rate changes 0.20% (0.50%) (0.70%)
Other changes in state deferred taxes 0.00% 0.00% 5.90%
Uncertain tax positions 4.70% 26.30% 0.00%
Stock-based compensation 5.10% (1.00%) (3.90%)
Other items 1.20% 3.00% 1.60%
Effective tax rate 26.30% (298.10%) 0.50%
Income Before Income Taxes, Current Year      
Valuation Allowance [Line Items]      
Reduction in valuation allowances   $ 7.5  
Income Tax Credit Carryforwards and Uncertain Tax Positions      
Valuation Allowance [Line Items]      
Reduction in valuation allowances   10.7  
Estimated Realizability of Deferred Tax Asset      
Valuation Allowance [Line Items]      
Reduction in valuation allowances   $ 85.6