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Summary of Significant Accounting Policies - Additional Information (Detail)
 1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended 14 Months Ended
Mar. 31, 2016
USD ($)
$ / shares
shares
Feb. 28, 2015
USD ($)
Apr. 30, 2013
USD ($)
Dec. 31, 2011
$ / shares
shares
Jun. 30, 2016
USD ($)
$ / shares
shares
Jun. 30, 2016
USD ($)
Employees
Segment
$ / shares
shares
Jun. 30, 2015
USD ($)
Dec. 31, 2014
USD ($)
shares
Dec. 31, 2013
shares
Dec. 31, 2012
shares
Dec. 31, 2011
$ / shares
shares
Aug. 31, 2014
USD ($)
May 06, 2016
shares
May 05, 2016
shares
Dec. 31, 2015
USD ($)
shares
Apr. 30, 2015
$ / shares
Class of Warrant or Right [Line Items]                                
Number of segments | Segment           1                    
Number of employees eliminated | Employees           50                    
Wind-down expense         $ 3,803,448 $ 3,803,448                    
Accumulated deficit         (463,732,581) (463,732,581)                 $ (456,686,634)  
Cash and cash equivalents         $ 2,448,761 $ 2,448,761 $ 29,929,140 $ 24,987,603             $ 12,110,565  
Common stock, shares authorized | shares         200,000,000 [1] 200,000,000 [1]             200,000,000 225,000,000 200,000,000 [1]  
Maturity period for market accounts, money market funds and investments           90 days                    
Term of warrant 5 years                              
Fair value of warrant liability         $ 591,037 $ 591,037                 $ 770,964  
Proceeds from issuance of stock units           7,259,613 $ 24,942,963                  
Warrant expiration period 2 years                              
Value of patent           0                    
Other Income         8,917,000                      
Gain on extinguishment of a loan         243,000                      
Accumulated other comprehensive income         47,274 47,274                 47,359  
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                            
Other Income         8,917,000                      
Gain on extinguishment of a loan         243,000                      
2011 Series A Warrant [Member]                                
Class of Warrant or Right [Line Items]                                
Fair value of warrant liability         57,000 57,000                    
Series A Warrants [Member]                                
Class of Warrant or Right [Line Items]                                
Number of securities callable by each warrant or right warrants | shares 0.50                              
Exercise price of warrants | $ / shares $ 3.60                              
Number of additional warrants issued | shares 166,473                              
Fair value of warrant liability         160,000 160,000                    
Series A Warrants [Member] | Common Stock [Member]                                
Class of Warrant or Right [Line Items]                                
Exercise price of warrants | $ / shares                               $ 8.40
Series B Warrants [Member]                                
Class of Warrant or Right [Line Items]                                
Number of securities callable by each warrant or right warrants | shares 0.75                              
Exercise price of warrants | $ / shares $ 5.04                              
Number of additional warrants issued | shares 249,709                              
Fair value of warrant liability         $ 374,000 $ 374,000                    
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         1          
Number of shares issued upon Series B warrants exercise | shares                   225,000            
December 2011 Financing [Member] | Series A Warrants [Member]                                
Class of Warrant or Right [Line Items]                                
Number of securities callable by each warrant or right warrants | shares         1 1                    
Term of warrant       5 years                        
Number of securities callable by warrants | shares       666,667             666,667          
Number of warrants exercised | shares               98,335 32,045 183,215            
Number of additional warrants issued | shares                   225,000            
Fair value of warrant liability         $ 57,286 $ 57,286                 511,594  
December 2011 Financing [Member] | Series A Warrants [Member] | Common Stock [Member]                                
Class of Warrant or Right [Line Items]                                
Exercise price of warrants | $ / shares       $ 16.80             $ 16.80          
Number of shares issued upon Series B warrants exercise | shares               98,335 32,045 183,215            
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       666,667             666,667          
Exercise price of warrants | $ / shares       $ 15.00             $ 15.00          
Number of warrants exercised | shares                   225,000            
Number of warrants expired | shares           441,667       441,667            
April 2015 Financing [Member] | Series A Warrants [Member] | Common Stock [Member]                                
Class of Warrant or Right [Line Items]                                
Exercise price of warrants | $ / shares $ 6.24       $ 6.24 $ 6.24                   $ 8.40
March 2016 Financing [Member] | Series A Warrants [Member]                                
Class of Warrant or Right [Line Items]                                
Fair value of warrant liability         $ 160,224 $ 160,224                 1,760,937  
March 2016 Financing [Member] | Series A Warrants [Member] | Common Stock [Member]                                
Class of Warrant or Right [Line Items]                                
Exercise price of warrants | $ / shares $ 3.60       $ 3.60 $ 3.60                    
March 2016 Financing [Member] | Series B Warrants [Member]                                
Class of Warrant or Right [Line Items]                                
Fair value of warrant liability         $ 373,527 $ 373,527                 $ 3,887,140  
Underwritten Public Offering [Member]                                
Class of Warrant or Right [Line Items]                                
Number of shares issued upon Series B warrants exercise | shares 2,222,250                              
Proceeds from issuance of stock units $ 8,000,000                              
Number of stock units issued, price per share | $ / shares $ 3.60                              
Over-Allotment-Option [Member]                                
Class of Warrant or Right [Line Items]                                
Number of securities callable by each warrant or right warrants | shares 416,672                              
Number of shares issued upon Series B warrants exercise | shares 333,338                              
Underwriting option exercise period 45 days                              
[1] * Adjusted for the 1-for-12 reverse stock split as discussed in Note 1.