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Income Taxes - Schedule of Reconciliation Between Statutory Federal Income Tax Rate and Effective Tax Rate (Detail)
12 Months Ended
Dec. 22, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]        
Tax at statutory federal rate 35.00% 34.00% 34.00% 34.00%
State tax   0.00% 0.00% 0.00%
Stock-based compensation expense   18.50% (7.90%) (4.60%)
Change in deferred tax assets due to rate change   43.90% 0.00% 0.00%
Change in valuation allowance due to rate change   (43.90%) 0.00% 0.00%
Net operating losses not benefitted   (43.80%) (12.90%) (21.70%)
Foreign net operating losses benefitted   (6.70%) (13.00%) (7.60%)
Orphan drug credit   (2.00%) 0.00% 0.00%
Other   (0.30%) (0.10%) (0.40%)
Total   (0.30%) 0.10% (0.30%)