Quarterly report pursuant to Section 13 or 15(d)

Research and Development Agreements - Schedule of Research and Development Revenues Disaggregated by Location (Detail)

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Research and Development Agreements - Schedule of Research and Development Revenues Disaggregated by Location (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Revenue Recognition [Line Items]        
Research and development revenue $ 944 $ 0 $ 1,175 $ 1,760
JAPAN        
Revenue Recognition [Line Items]        
Research and development revenue 932 0 1,095 1,260
UNITED STATES        
Revenue Recognition [Line Items]        
Research and development revenue $ 12 $ 0 $ 80 $ 500