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Changes in Deferred Revenue (Detail) - USD ($)
3 Months Ended
Mar. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Disaggregation Of Revenue [Line Items]      
Deferred revenue $ 4,327,000   $ 1,090,000
Fair value of deferred revenue acquired in acquisition, net of recognition 2,637,000    
Deferral of revenue billed in current period, net of recognition 895,000    
Recognition of revenue deferred in prior periods $ (293,000)    
Topic 606      
Disaggregation Of Revenue [Line Items]      
Deferred revenue   $ 1,088,000  
Topic 606 | Impact of Adoption of Topic 606      
Disaggregation Of Revenue [Line Items]      
Deferred revenue   $ (2,000)