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Valuation and Qualifying Accounts (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Allowance for Uncollectible Trade Accounts [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Year $ 6,910,000 $ 6,728,000 $ 5,996,000
Charged to Costs & Expenses 135,000 180,000 54,000
Charged to Other Accounts [1] 1,242,000 5,000 680,000
Deductions [2] (7,000) (3,000) (2,000)
Balance at End of Year 8,280,000 6,910,000 6,728,000
Deferred tax valuation allowance [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Year 4,713,000 3,128,000 1,681,000
Charged to Costs & Expenses 1,805,000 1,585,000 1,447,000
Balance at End of Year $ 6,518,000 $ 4,713,000 $ 3,128,000
[1] Expected customer account adjustments charged to revenue and write-offs, net of recoveries
[2] Represents bad debt recoveries