Note 10 - Income Taxes (Details) - Income Tax Reconciliation	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Note 10 - Income Taxes (Details) - Income Tax Reconciliation [Line Items]
Federal statutory rate	35.00%	35.00%	35.00%
Changes from statutory rates resulting from:
Company-owned life insurance	(1.60%)	(1.90%)	(1.80%)
Uncertain tax position	0.00%	(0.30%)	(0.50%)
Other, net	(0.20%)	0.40%	(0.60%)
Effective tax rate on pretax income	26.10%	27.00%	24.80%
Interest Income [Member]
Changes from statutory rates resulting from:
Tax-exempt interest	(5.80%)	(5.20%)	(6.30%)
Premium income not subject to tax	(5.80%)	(5.20%)	(6.30%)
Interest to Carry Tax Exempt Obligations [Member]
Changes from statutory rates resulting from:
Nondeductible interest to carry tax-exempt obligations	0.20%	0.20%	0.30%
Other Income [Member]
Changes from statutory rates resulting from:
Tax-exempt interest	(1.50%)	(1.20%)	(1.30%)
Premium income not subject to tax	(1.50%)	(1.20%)	(1.30%)