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Note 6 - Revenues (Details Textual) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Contract with Customer, Asset, after Allowance for Credit Loss, Total $ 0 $ 0
Revenue, Remaining Performance Obligation, Amount 468,200 $ 443,200
Increase (Decrease) in Contract Liabilities 25,000  
Contract With Customer Liability, Billings Paid in Advance 418,500  
Contract with Customer, Liability, Revenue Recognized $ 393,500  
Revenue Remaining Performance Obligation, Current Percentage 99.00% 99.00%
Contract with Customer, Liability, Noncurrent $ 73,800 $ 63,700