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Allowance for Doubtful Accounts - Schedule of Allowance for Doubtful Accounts (Detail) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Receivables [Abstract]    
Balance at Beginning of Period $ 5,907 $ 5,918
Charged to Expense 8,669 7,830
Deductions (8,483) (8,179)
Balance at End of Period $ 6,093 $ 5,569