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Segment Information - Geographical Segment Revenue (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Total $ 4,858 $ 8,677 $ 5,587 $ 6,091 $ 5,180 $ 5,576 $ 5,457 $ 5,580 $ 25,213 $ 21,793 $ 22,189
Domestic [Member]                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Total                 5,116 4,776 6,304
International [Member]                      
Revenues from External Customers and Long-Lived Assets [Line Items]                      
Total                 $ 20,097 $ 17,017 $ 15,885