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Financial instruments - Fair Value of Financial Instruments Measured At Fair Value on a Recurring Basis (Detail) - USD ($)
$ in Thousands
Sep. 30, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments $ 59,711 $ 34,956
Long-term investments 16,666 38,458
Bank loan warrant derivative liabilities   (124)
Fair Value of Financial instruments 76,377 73,290
U.S. Treasury Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 59,711 32,953
Long-term investments 16,666 38,458
U.S. Government Agency Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments   2,003
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value of Financial instruments 76,377 73,414
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments 59,711 32,953
Long-term investments $ 16,666 38,458
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Government Agency Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term investments   2,003
Significant Un-Observable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Bank loan warrant derivative liabilities   (124)
Fair Value of Financial instruments   $ (124)