Note 10 - Income Taxes - Income Tax Reconciliation (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Federal statutory rate	35.00%	35.00%	35.00%
Changes from statutory rates resulting from:
Company-owned life insurance	(1.50%)	(1.60%)	(1.90%)
Uncertain tax position			(0.30%)
Other, net	0.10%	(0.20%)	0.40%
Effective tax rate on pretax income	27.90%	26.10%	27.00%
Interest Income [Member]
Changes from statutory rates resulting from:
Tax-exempt interest	(4.60%)	(5.80%)	(5.20%)
Interest to Carry Tax-exempt Obligations [Member]
Changes from statutory rates resulting from:
Nondeductible interest to carry tax-exempt obligations	0.20%	0.20%	0.20%
Other Income [Member]
Changes from statutory rates resulting from:
Tax-exempt interest	(1.30%)	(1.50%)	(1.20%)