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Reconciling Items from Income Tax Computed at Statutory Federal Rate (Detail)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Components Of Income Tax Expense Benefit Continuing Operations [Abstract]      
Federal income tax at statutory rate 35.00% 35.00% 35.00%
State income taxes, net of federal benefits 2.90% 5.40% 4.50%
Research and other credits (5.80%) (1.20%) (1.30%)
Permanent deductions (18.00%) (4.00%) (2.20%)
Valuation allowance (14.10%) (35.20%) (36.00%)
Effective tax rate 0.00% 0.00% 0.00%