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Summary of Significant Accounting Policies - Additional Information (Detail)
 1 Months Ended 3 Months Ended 12 Months Ended 14 Months Ended
Aug. 31, 2015
shares
Apr. 30, 2015
$ / shares
shares
Feb. 28, 2015
USD ($)
Jul. 31, 2014
$ / shares
shares
Oct. 31, 2013
$ / shares
shares
Apr. 30, 2013
USD ($)
Dec. 31, 2011
$ / shares
shares
Dec. 31, 2014
USD ($)
shares
Dec. 31, 2015
USD ($)
Segment
Employees
$ / shares
shares
Dec. 31, 2014
USD ($)
shares
Nov. 30, 2014
USD ($)
Dec. 31, 2013
shares
Dec. 31, 2012
shares
Dec. 31, 2011
$ / shares
shares
Aug. 31, 2014
USD ($)
Apr. 01, 2009
USD ($)
Class of Warrant or Right [Line Items]                                
Number of segments | Segment                 1              
Accumulated deficit               $ (420,271,608) $ (456,686,634) $ (420,271,608)            
Maturity period for market accounts, money market funds and investments                 90 days              
Impairment of goodwill                   1,910,062 $ 1,910,062          
Other intangible assets, net               356,889 $ 45,816 356,889            
Impairment amount of intangible asset               530,000 239,000 530,000            
Value of patent                 0              
Fair value of certain lab equipment                 $ 0 0            
Term of loan                 3 years              
Annual interest rate on loan                 6.00%              
Final fee payment at the end of loan term                 $ 1,000,000              
Exercise price of warrants | $ / shares   $ 0.85   $ 2.17 $ 1.80                      
Initial carrying amount assigned to loan, net of discount                 9,512,000              
Fair value allocated to warrant                 388,000              
Note issuance costs                 $ 117,000              
Deferred financing costs, discount and accretion percentage                 9.00%              
Number of securities callable by each warrant or right warrants | shares                 1              
Number of shares issued upon Series B warrants exercise | shares   35,715,000   11,299,435 12,845,500                      
Fair value of warrant liability               1,684,551 $ 770,964 1,684,551            
Accrued expenses and accrued liabilities               $ 4,869,710 $ 5,731,596 $ 4,869,710            
Employee Severance [Member]                                
Class of Warrant or Right [Line Items]                                
Number of employees eliminated | Employees                 17              
Percentage of employees eliminated                 25.00%              
Severance and related expenses                 $ 392,000              
Accrued expenses and accrued liabilities                 392,000              
Silicon Valley Bank Loan Agreement [Member]                                
Class of Warrant or Right [Line Items]                                
Proceeds from loan           $ 9,900,000                    
Cash discount           100,000     $ 100,000              
Term of loan                 3 years              
Annual interest rate on loan                 6.00%              
Final fee payment at the end of loan term                 $ 1,000,000              
Initial carrying amount assigned to loan, net of discount                 9,512,000              
Fair value allocated to warrant                 388,000              
Note issuance costs                 $ 117,000              
Deferred financing costs, discount and accretion percentage                 9.00%              
Minimum cash proceeds from investors required to release pledged restricted money market account                 $ 18,000,000              
Silicon Valley Bank Loan Agreement [Member] | Restricted Money Market Account [Member]                                
Class of Warrant or Right [Line Items]                                
Debt instrument, collateral amount                 2,422,500              
CIRM [Member]                                
Class of Warrant or Right [Line Items]                                
Approval of fund by California Institute for Regenerative Medicine (CIRM)           19,300,000                    
Aggregate proceeds from disbursement of the loan                             $ 9,600,000  
Forgiveness of loan principal                 8,917,000              
Accrued interest forgiven                 243,000              
Repayment of aggregate loan proceeds received     $ 679,000                          
Silicon Valley Bank [Member]                                
Class of Warrant or Right [Line Items]                                
Proceeds from loan           9,900,000                    
Cash discount           $ 100,000                    
Common Stock [Member]                                
Class of Warrant or Right [Line Items]                                
Number of securities callable by warrants | shares               1,180,015   1,180,015            
Number of securities callable by each warrant or right warrants | shares   0.75   0.85 0.5                      
Warrants [Member]                                
Class of Warrant or Right [Line Items]                                
Fair value of warrant liability                 $ 771,000              
Marketable debt securities, current [Member] | Minimum [Member]                                
Class of Warrant or Right [Line Items]                                
Investments with maturities                 365 days or less              
Marketable securities, non-current [Member] | Maximum [Member]                                
Class of Warrant or Right [Line Items]                                
Investments with maturities                 Greater than 365 days              
Stem Cell Sciences Holdings Ltd [Member]                                
Class of Warrant or Right [Line Items]                                
Aggregate purchase price                               $ 5,135,000
Percentage of purchase price allocated to goodwill               42.00%                
Impairment of goodwill               $ 1,910,000                
Series A Warrants [Member]                                
Class of Warrant or Right [Line Items]                                
Number of securities callable by each warrant or right warrants | shares                 1              
Series A Warrants [Member] | Common Stock [Member]                                
Class of Warrant or Right [Line Items]                                
Exercise price of warrants | $ / shares   $ 0.70                            
Silicon Valley Bank Loan Agreement Warrant [Member]                                
Class of Warrant or Right [Line Items]                                
Term of warrant                 10 years              
Exercise price of warrants | $ / shares                 $ 1.7034              
Warrant expiration period                 April 2023              
Silicon Valley Bank Loan Agreement Warrant [Member] | Common Stock [Member]                                
Class of Warrant or Right [Line Items]                                
Number of securities callable by warrants | shares                 293,531              
December 2011 Financing [Member] | Common Stock [Member]                                
Class of Warrant or Right [Line Items]                                
Number of securities callable by each warrant or right warrants | shares             1   1         1    
Number of shares issued upon Series B warrants exercise | shares                         2,700,000      
December 2011 Financing [Member] | Series A Warrants [Member]                                
Class of Warrant or Right [Line Items]                                
Term of warrant             5 years                  
Number of securities callable by warrants | shares             8,000,000             8,000,000    
Number of securities callable by each warrant or right warrants | shares                 1              
Number of warrants exercised | shares                   1,180,015   384,534 2,198,571      
Number of warrants issued upon Series B warrants exercise | shares                         2,700,000      
December 2011 Financing [Member] | Series A Warrants [Member] | Common Stock [Member]                                
Class of Warrant or Right [Line Items]                                
Exercise price of warrants | $ / shares             $ 1.40             $ 1.40    
Number of shares issued upon Series B warrants exercise | shares                   1,180,015   384,534 2,198,571      
December 2011 Financing [Member] | Series B Warrants [Member]                                
Class of Warrant or Right [Line Items]                                
Term of warrant             90 days             90 days    
Number of securities callable by warrants | shares             8,000,000             8,000,000    
Exercise price of warrants | $ / shares             $ 1.25             $ 1.25    
Number of warrants exercised | shares                         2,700,000      
Number of warrants expired | shares                 5,300,000       5,300,000      
April 2015 Financing [Member] | Series A Warrants [Member] | Common Stock [Member]                                
Class of Warrant or Right [Line Items]                                
Exercise price of warrants | $ / shares   $ 0.70             $ 0.52              
July 2014 Financing [Member]                                
Class of Warrant or Right [Line Items]                                
Number of warrants expired | shares 9,604,520