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Long-term Investments - Additional Information (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Mar. 04, 2015
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]            
Fair value of the debt security   $ 9,000 $ 16,100 $ 16,100    
Unrealized gain (loss) on debt securities       3,830 $ (1,744) $ (3,348)
Amortized cost basis in debt security     9,000 9,000    
Other-than-temporary impairment on debt securities   5,600   5,585 $ 2,727  
Bone Biologics Inc [Member]            
Investments Equity Securities [Abstract]            
Value of common stock held     $ 2,500 $ 2,500    
Warrants held for share purchases     458 458    
Weighted average exercise price of warrant     $ 1.18 $ 1.18    
Warrants exercisable period       7 years    
Warrant expiration term       between 2020 and 2021    
Warrants expiration start year       2020    
Warrants expiration end year       2021    
eNeura Note [Member] | eNeura Inc [Member]            
Debt Instrument [Line Items]            
Non-refundable fees paid $ 300          
Accrued interest rate on promissory note 8.00%          
Maturity description of Promissory note       The eNeura Note will mature on March 4, 2019    
Debt Instrument, Maturity Date       Mar. 04, 2019    
eNeura Note [Member] | eNeura Inc [Member] | Convertible Debt Securities            
Debt Instrument [Line Items]            
Principal amount of promissory note $ 15,000          
eNeura Note [Member] | eNeura Inc [Member] | Debt Securities [Member]            
Debt Instrument [Line Items]            
Unrealized gain (loss) on debt securities   $ (3,200) $ 7,100      
eNeura Note [Member] | eNeura Inc [Member] | Preferred Stock [Member]            
Debt Instrument [Line Items]            
Conversion price per share     $ 7.30 $ 7.30    
eNeura Note [Member] | eNeura Inc [Member] | Minimum [Member]            
Debt Instrument [Line Items]            
Third party acquisition     50.00% 50.00%