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Note 10 - Employment Benefit and Retirement Plans - Employment Benefit and Retirement Plans Valuation (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Director Retirement Plan [Member]
Number of participants	6	6
Director Retirement Plan [Member] \| Retirees [Member]
Number of participants	4	4
Director Retirement Plan [Member] \| Active Employees Not Yet Eligible [Member]
Number of participants	2	2
Postretirement Health Coverage [Member]
Number of participants	6	6
Postretirement Health Coverage [Member] \| Retirees [Member]
Number of participants	3	3
Postretirement Health Coverage [Member] \| Active Employees Not Yet Eligible [Member]
Number of participants	3	3
Postretirement Health Coverage [Member] \| Active Employees Fully Eligible [Member]
Number of participants