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Reconciling Items from Income Tax Computed at Statutory Federal Rate (Detail)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Reconciliation of Effective Income Tax Rate [Line Items]      
Federal income tax statutory rate 34.00% 34.00% 34.00%
State income taxes, net of federal benefits 4.50% 6.00% 5.80%
Revalued preferred stock warrants 0.90% (2.30%) (6.50%)
Incentive stock option compensation (1.90%) (2.40%) (1.10%)
Research tax credits   0.40% 3.40%
Other (0.10%) (0.10%) (0.20%)
Valuation allowance (37.40%) (35.60%) (35.40%)
Effective income tax rate 0.00% 0.00% 0.00%