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Income Taxes - Schedule of Reconciliation Between Statutory Federal Income Tax Rate and Effective Tax Rate (Detail)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]      
Tax at statutory federal rate 21.00% 21.00% 34.00%
State tax 0.00% 0.00% 0.00%
Stock-based compensation expense 6.30% 14.50% 18.50%
Change in deferred tax assets due to rate change 0.00% 0.00% 43.90%
Change in valuation allowance due to rate change 0.00% 0.00% (43.90%)
Net operating losses not benefitted (2.90%) (23.20%) (43.80%)
Foreign net operating losses not benefitted (21.70%) (11.60%) (6.70%)
Orphan drug credit 0.00% 0.00% (2.00%)
Deduction limitation on executive compensation (2.50%) (0.50%) 0.00%
Other (0.60%) (0.60%) (0.30%)
Total (0.40%) (0.40%) (0.30%)