Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Deferred tax liability	$ 397,000	$ 397,000
Deferred income tax provision	$ 0	85,000	$ 320,000
Income tax expenses related to unrealized gain on a marketable equity security		$ 61,000
Statutory federal income tax rate	34.00%	34.00%	34.00%
Total income tax provision	$ 0	$ 53,000	$ 24,000
Increase in valuation allowance of net deferred tax assets	11,400,000	7,200,000	8,900,000
Unrecognized tax benefits	6,982,000	6,965,000	5,702,000
Interest and penalty expense related to unrecognized tax benefits	0	0	0
State [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards for income tax purposes	$ 215,900,000
Tax carryforwards expire date	2017 through 2036
Federal [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards for income tax purposes	$ 327,200,000
Tax carryforwards expire date	2019 through 2036
Research [Member] | State [Member]
Income Tax Contingency [Line Items]
Research and development tax credits	$ 12,200,000
Research [Member] | Federal [Member]
Income Tax Contingency [Line Items]
Tax carryforwards expire date	2018 through 2036
Research and development tax credits	$ 11,100,000
Interest and other income (expenses) [Member]
Income Tax Contingency [Line Items]
Deferred income tax provision	$ 0	$ 53,000	$ 24,000