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Income Taxes - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Income Taxes [Line Items]          
Income tax expense (benefit) $ (109,000)   $ 0 $ 1,891,000 $ 0
Effective tax rate       2.30% 0.00%
Gain from sale of PRV asset       $ 125,000,000  
Gilead [Member]          
Income Taxes [Line Items]          
Gain from sale of PRV asset   $ 125,000,000