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Valuation and Qualifying Accounts - Schedule of Valuation and Qualifying Accounts (Detail) - Allowance for Doubtful Accounts Receivable [Member] - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Year $ 5,918 $ 5,137 $ 6,472
Charged to Expense 18,478 18,639 16,620
Deductions (18,489) (17,858) (17,955)
Balance at End of Year $ 5,907 $ 5,918 $ 5,137