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Schedule II - Valuation and Qualifying Accounts (Detail) - Allowance for Doubtful Accounts [Member] - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Period $ 153,000 $ 171,000 $ 183,000
Charge (Recovery) to Costs and Expenses 6,000 (22,000) 18,000
Other Charges, Deductions   4,000 (30,000)
Balance at End of Period $ 159,000 $ 153,000 $ 171,000