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Income Taxes - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Taxes [Line Items]        
Income tax benefit (provision) $ (69,000) $ (842,000) $ (482,000)  
U.S. federal corporate tax rate 21.00% 35.00%    
Benefit related to remeasurement of deferred tax assets and liabilities as result of filing tax return $ 200,000      
Net operating loss carryovers 58,661,000 $ 28,913,000    
Increase in valuation allowance 63,300,000 20,100,000 8,500,000  
Gross unrecognized tax benefit 10,219,000 6,207,000 2,343,000 $ 1,280,000
Unrecognized tax benefit that could affect annual effective tax rate if recognized 5,800,000      
Subsidiaries Outside of Singapore [Member]        
Income Taxes [Line Items]        
Cash held by subsidiaries 53,000,000 48,800,000    
Maximum [Member]        
Income Taxes [Line Items]        
Interest and penalties related to uncertain tax positions 100,000 100,000    
Federal [Member]        
Income Taxes [Line Items]        
Net operating loss carryforwards 82,600,000      
Net operating loss carryforwards available to offset future income tax liabilities indefinitely $ 80,600,000      
Net operating loss carryforwards available to offset future income tax liabilities   2,000,000    
Operating Loss Carry forward expiration year 2037      
Research and development tax credit carryforward 2031      
Federal [Member] | Research and development        
Income Taxes [Line Items]        
Research and development tax credit carryforward $ 6,200,000 2,800,000    
Federal [Member] | Orphan Drug Credits [Member]        
Income Taxes [Line Items]        
Research and development tax credit carryforward $ 5,000,000      
U.S.Orphan drug credit carryforward expiration year 2037      
State [Member]        
Income Taxes [Line Items]        
Research and development tax credit carryforward 2031      
State [Member] | Research and development        
Income Taxes [Line Items]        
Research and development tax credit carryforward $ 1,800,000 1,100,000    
Singapore [Member]        
Income Taxes [Line Items]        
Income tax benefit (provision) $ 0 $ 0 $ 0  
U.S. federal corporate tax rate 17.00% 17.00% 17.00%  
Net operating loss carryovers $ 161,300,000 $ 149,200,000    
Tax years open to examination 2012      
United States [Member]        
Income Taxes [Line Items]        
Income tax benefit (provision) $ 0      
Tax years open to examination 2012      
Japan [Member]        
Income Taxes [Line Items]        
Income tax benefit (provision) $ 0   $ 0  
Net operating loss carryovers $ 2,900,000 4,100,000    
Operating loss carryforwards expiration year 2023      
Tax years open to examination 2008      
UK [Member]        
Income Taxes [Line Items]        
Income tax benefit (provision) $ 0 0    
Net operating loss carryovers $ 46,400,000 10,500,000    
Tax years open to examination 2017      
Ireland [Member]        
Income Taxes [Line Items]        
Income tax benefit (provision) $ 0 $ 0 $ 0