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Note 9 - Employee Retirement Plans - Costs of Retirement Plans (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Service cost $ 112 $ 427 $ 294
Interest cost $ 2,907 $ 3,751 3,605
Postretirement Health Care And Life Insurance [Member]      
Service cost     61
Interest cost     117
Amortization of prior service cost     (299)
Recognized actuarial loss     42
Net curtailment gain     (4,005)
Net periodic pension cost     $ (4,084)