Note 10 - Income Taxes - Income Tax Reconciliation (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Federal statutory rate	35.00%	35.00%	35.00%
Changes from statutory rates resulting from:
Revaluation of deferred income taxes resulting from changes in statutory tax rates	24.10%
Company-owned life insurance	(1.60%)	(1.50%)	(1.60%)
Other, net	(0.10%)	0.10%	(0.20%)
Effective tax rate on pretax income	52.00%	27.90%	26.10%
Interest Income [Member]
Changes from statutory rates resulting from:
Tax-exempt interest	(4.10%)	(4.60%)	(5.80%)
Interest to Carry Tax-exempt Obligations [Member]
Changes from statutory rates resulting from:
Nondeductible interest to carry tax-exempt obligations	0.10%	0.20%	0.20%
Other Income [Member]
Changes from statutory rates resulting from:
Tax-exempt interest	(1.40%)	(1.30%)	(1.50%)