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Schedule II - Valuation and Qualifying Accounts (Detail) - Allowance for Doubtful Accounts [Member] - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Period $ 159,000 $ 153,000 $ 171,000
Charge (Recovery) to Costs and Expenses 65,000 6,000 (22,000)
Other Charges, Deductions [1]     4,000
Balance at End of Period $ 224,000 $ 159,000 $ 153,000
[1] Reflects uncollectible accounts written off, net of recoveries.