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Income Taxes - Reconciliation Between Statutory Federal Income Tax Rate and Effective Income Tax Rate (Detail)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Income Taxes      
U.S. Federal statutory tax rate 34.00% 34.00% 34.00%
State taxes 4.60% 3.70% 4.60%
Research credit 1.00% 5.60% 7.40%
Orphan drug credit 9.00% 11.50% 8.70%
Stock-based compensation (0.70%) (1.80%) (1.30%)
Other     2.40%
Change in valuation allowance (47.90%) (53.00%) (55.80%)