Significant Accounting Policies - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014
Summary Of Significant Accounting Policies [Line Items]
Investment in savings account	$ 18,800,000
Percentage of interest recognized from savings account investment	0.10%
Interest income from savings account investment	5,194
Maximum
Summary Of Significant Accounting Policies [Line Items]
Deposit insured by the Federal Deposit Insurance Corporation	250,000
Option and License Agreement
Summary Of Significant Accounting Policies [Line Items]
Proceeds from up front cash payment	5,000,000
Licensing milestone	25,000,000
Option and License Agreement, deferred revenue	5,000,000
Option and License Agreement | Licensing fee if Merck is unable to advance directly to Phase III trials of Tcelna
Summary Of Significant Accounting Policies [Line Items]
Licensing milestone	$ 15,000,000