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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 |  |  |  |
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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.

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