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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 Disclosure [Line Items]        
Statutory income tax rate   35.00%    
Valuation allowance, increase (decrease) amount   $ (169,300,000) $ 52,500,000 $ 17,300,000
Valuation allowance related to stock-based compensation included in income tax expense   35,600,000 35,600,000  
Additional income taxes for remaining undistributed foreign earnings   0    
Income tax research credits   8,038,000 $ 4,543,000 $ 4,529,000
Federal orphan drug credits   $ 17,700,000    
Federal orphan drug credits, expiration year   2026    
Unrecognized tax benefits, period may increase or decrease due to tax examination   Next twelve months    
Federal [Member]        
Income Taxes Disclosure [Line Items]        
Net operating loss carryforward for income tax   $ 1,610,300,000    
Net operating loss carryforward, expiration year   2018    
Income tax research credits   $ 64,000,000    
Income tax research credits, expiration year   2019    
State [Member]        
Income Taxes Disclosure [Line Items]        
Net operating loss carryforward for income tax   $ 474,000,000    
Net operating loss carryforward, expiration year   2019    
Income tax research credits   $ 33,300,000    
Scenario Forecast [Member]        
Income Taxes Disclosure [Line Items]        
Statutory income tax rate 21.00%