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Allowance for Doubtful Accounts Included in Accounts Receivable Net (Detail) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Valuation Allowance [Line Items]      
Allowance for Doubtful Accounts $ 1.5 $ 1.1 $ 1.2