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Summary of Significant Accounting Policies - Reconciliation of Allowance for Doubtful Accounts (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Beginning balance $ 5,728 $ 5,645  
Provision 2,335 2,398 $ 992
Write-off of uncollectible amounts, net of recoveries (3,616) (2,315) (1,807)
Ending balance $ 4,447 $ 5,728 $ 5,645