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Summary of Significant Accounting Policies - Additional Information (Detail)
 1 Months Ended 12 Months Ended
Jan. 31, 2014
USD ($)
Dec. 31, 2017
USD ($)
Segment
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Jan. 01, 2018
USD ($)
Jan. 01, 2017
USD ($)
May 24, 2016
USD ($)
Summary Of Significant Accounting Policies [Line Items]              
Long-term marketable securities, minimum original maturity term     1 year        
Investment fund   $ 0          
Long-term marketable securities     $ 0        
Gain (loss) on investments   0 0 $ 0      
Fair value of other assets   0          
Fair value of other liabilities   0          
Accrued liability contingent consideration     6,119,000        
Reserves for excess and obsolete inventory   $ 455,000 $ 435,000        
Stock options, outstanding | shares   734,940 882,748        
Stock options, weighted average exercise price | $ / shares   $ 20.80 $ 16.88        
Common stock excluded from calculation of diluted earnings per share | shares   317,923 381,686 196,209      
Number of operating segment | Segment   1          
Number of reporting segment | Segment   2          
Goodwill impairment   $ 0 $ 0 $ 0      
Amortization expense   6,215,000 2,052,000 $ 1,600,000      
Restricted Cash     450,000        
ASU No. 2016-09 | U. S. Federal and State Jurisdiction              
Summary Of Significant Accounting Policies [Line Items]              
Net operating loss carry forwards           $ 5,100,000  
Increase in valuation allowance           5,100,000  
Impact of adopting ASU 2016-09 on retained earnings           $ 0  
2.125% Convertible Senior Notes due 2021              
Summary Of Significant Accounting Policies [Line Items]              
Principal amount   $ 11,000         $ 115,000,000
Notes, due date   Jun. 01, 2021          
Notes, frequency of periodic payment   Semi-annually          
Notes, date of first required payment   Dec. 01, 2016          
Total convertible senior notes   $ 99,250,000 $ 95,272,000        
Fair value of convertible senior notes   $ 149,500,000          
Minimum              
Summary Of Significant Accounting Policies [Line Items]              
Short-term marketable securities, minimum original maturity term     90 days        
Restricted Stock Units (RSUs)              
Summary Of Significant Accounting Policies [Line Items]              
Restricted stock units, outstanding | shares   505,235 353,838        
Restricted Stock Units (RSUs) | Awards Granted to Non-Executive Level Employees              
Summary Of Significant Accounting Policies [Line Items]              
Estimated forfeiture rates   8.00%          
Restricted Stock Units (RSUs) | Awards Granted to Executive Level Employees              
Summary Of Significant Accounting Policies [Line Items]              
Estimated forfeiture rates   3.00%          
Option To Purchase Common Stock              
Summary Of Significant Accounting Policies [Line Items]              
Stock options, outstanding | shares   734,940 1,236,586 1,240,935      
Stock options, weighted average exercise price | $ / shares   $ 20.80 $ 12.05 $ 10.44      
Employee Stock Option | Awards Granted to Non-Executive Level Employees              
Summary Of Significant Accounting Policies [Line Items]              
Estimated forfeiture rates   8.00%          
Employee Stock Option | Awards Granted to Executive Level Employees              
Summary Of Significant Accounting Policies [Line Items]              
Estimated forfeiture rates   3.00%          
Non-Employee Directors              
Summary Of Significant Accounting Policies [Line Items]              
Estimated forfeiture rates   0.00%          
BioMarin Pharmaceutical, Inc. | Asset Purchase Agreement              
Summary Of Significant Accounting Policies [Line Items]              
Potential milestone payments to be received $ 160,000,000            
Provision for refund $ 0            
BioMarin Pharmaceutical, Inc. | Clinical Development | Asset Purchase Agreement              
Summary Of Significant Accounting Policies [Line Items]              
Percentage relate to clinical development from Milestone payment 37.00%            
BioMarin Pharmaceutical, Inc. | Initial Commercial Sales | Asset Purchase Agreement              
Summary Of Significant Accounting Policies [Line Items]              
Percentage relate to clinical development from Milestone payment 63.00%            
Subsequent Event | ASU No. 2016-18              
Summary Of Significant Accounting Policies [Line Items]              
Restricted Cash         $ 450,000