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Schedule V - Valuation Allowances and Qualifying Accounts (Detail) - Allowance for Doubtful Accounts [Member] - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Valuation and Qualifying Accounts Disclosure [Line Items]      
Beginning Balance $ 527 $ 344 $ 357
Charges to Earnings 505 397 395
Deductions 352 214 408
Ending Balance $ 680 $ 527 $ 344