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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 ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2012
USD ($)
a
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, Settlements, Benefit Obligation $ 9,300   $ 10,103  
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss), Net Actuarial Gain (Loss), before Tax     22,300 28,700  
Defined Benefit Plan, Additional Security to Support Unfunded Liabilities         $ 18,700
Defined Benefit Plan, Area of Real Estate Property Pledged as Security | a         6,800
Defined Benefit Plan, Period for Release of Real Estate Property Pledged as Security         5 years
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year     0    
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year     $ 0 $ 0  
Scenario, Forecast [Member]          
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss), Net Actuarial Gain (Loss), before Tax   $ 800