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Summary of significant accounting policies - Additional Information (Detail) - USD ($)
$ in Thousands
Jun. 30, 2019
Jan. 01, 2019
Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Operating lease, right-of-use asset $ 10,271    
Operating lease liability 11,831    
Deferred revenue 646   $ 444
Unbilled revenue in accounts receivable $ 23,740   $ 25,545
ASC 842      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Operating lease, right-of-use asset   $ 10,866  
Operating lease liability   $ 11,138