Accrued Expenses (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Accounts Payable And Accrued Liabilities [Line Items]
Research and development related	$ 14,407	$ 4,217
Legal and accounting fees	323	507
Deferred rent	866	831
Accrued offering costs	66	1,506
Other	298	338
Total accrued and other liabilities	$ 15,960	$ 7,399