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Note 9 - Pension and Other Postretirement Benefit Plans (Details Textual) - USD ($)
 6 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Directors' Plan [Member]    
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year   $ 300,000
Defined Benefit Plan, Plan Assets, Contributions by Employer $ 72,000  
Other Postretirement Benefit Plans [Member]    
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year   $ 200,000
Defined Benefit Plan, Plan Assets, Contributions by Employer $ 41,000