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Valuation and Qualifying Accounts (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Allowance for Uncollectible Trade Accounts [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Year $ 5,996,000 $ 3,463,000 $ 2,906,000
Charged to Costs & Expenses 54,000 457,000 46,000
Charged to Other Accounts [1] 680,000 2,079,000 (13,000)
Deductions [2] (2,000) (3,000) 524,000
Balance at End of Year 6,728,000 5,996,000 3,463,000
Deferred tax valuation allowance [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Year 1,681,000 456,000 108,000
Charged to Costs & Expenses 1,447,000 1,225,000 348,000
Balance at End of Year $ 3,128,000 $ 1,681,000 $ 456,000
[1] Expected customer account adjustments charged to revenue and write-offs, net of recoveries
[2] Represents bad debt recoveries