Reconciliation of Effective Tax Rate for the Provision for Income Taxes (Detail)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Effective Tax Rate Reconciliation [Line Items]
Federal statutory income tax rate	34.00%	34.00%	34.00%
State income taxes, net of federal benefit	0.40%	22.80%	(3.80%)
Federal and state research credits	3.40%	0.80%	(5.70%)
Stock based compensation	(0.20%)	0.40%	1.30%
Other	(0.50%)	(0.10%)	0.60%
Change in valuation allowance	(37.10%)	(57.90%)	(26.40%)
Effective income tax rate	0.00%	0.00%	0.00%