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Summary of Significant Accounting Policies - Summary of Allowance for Doubtful Accounts Activity (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounting Policies [Abstract]      
Allowance for doubtful accounts, Beginning Balance $ 2,087 $ 2,087  
Provision for doubtful accounts 1,480 1,892 $ 2,149
Write-offs, net of recoveries (1,647) (1,892)  
Allowance for doubtful accounts, Ending Balance $ 1,920 $ 2,087 $ 2,087