Organization - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Subsidiary	Dec. 31, 2015 USD ($)
Organization [Line Items]
Number of subsidiaries | Subsidiary	2
Cash, cash equivalents and marketable securities	$ 23,300,000
Working capital	16,000,000
Accumulated deficit	(521,916,000)	$ (495,029,000)
Loans Payable | Hercules May 2016 Loan Agreement
Organization [Line Items]
Unrestricted and unencumbered cash and cash equivalents	25,000,000
Loans Payable | Hercules May 2016 Loan Agreement | Financial covenant | Minimum
Organization [Line Items]
Unrestricted and unencumbered cash and cash equivalents	$ 10,000,000