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Schedule II - Valuation and Qualifying Accounts (Detail) - Allowance for Doubtful Accounts [Member] - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Period $ 59,000 $ 224,000 $ 159,000
Charge (Recovery) to Costs and Expenses 23,000 (144,000) 65,000
Other Charges, Deductions [1]   (21,000)  
Balance at End of Period $ 82,000 $ 59,000 $ 224,000
[1] Reflects uncollectible accounts written off, net of recoveries.