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Note 6 - Accrued Retirement Benefits (Details Textual)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2016
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Defined Benefit Plan, Number of Participants Offered a One-time Lump Sum Payment 500    
Defined Benefit Plan, Number of Participants Who Accepted a One-time Lump Sum Payment 325    
Defined Benefit Plan, Benefit Obligation, Payment for Settlement   $ 10,103
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax $ (22,300) (20,200) (22,300)
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year   700  
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year   0  
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year 0 $ 0 $ 0
Pension Plan [Member] | Qualified Plan [Member]      
Defined Benefit Plan, Benefit Obligation, Payment for Settlement 9,300    
Supplemental Employee Retirement Plan [Member] | Nonqualified Plan [Member]      
Defined Benefit Plan, Benefit Obligation, Payment for Settlement $ 800