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Income Taxes - Reconciliation Between Statutory Federal Income Tax Rate and Effective Income Tax Rate (Detail)
12 Months Ended
Jan. 01, 2018
Dec. 22, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Reconciliations between the statutory federal income tax rate and the effective income tax rate          
U.S. Federal statutory tax rate 21.00% 35.00% 21.00% 34.00% 34.00%
State taxes     5.70% 4.40% 4.60%
Research credit     0.80% 1.30% 1.00%
Orphan drug credit     10.40% 11.50% 9.00%
Other     0.60% 0.30%  
Stock-based compensation     (0.10%) (0.70%) (0.70%)
Change in valuation allowance     (38.40%) (5.10%) (47.90%)
Effective change due to corporate tax rate reduction       (45.70%)