Note 3 - Comprehensive Income - Components of Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrealized holding losses on available-for-sale securities	$ (691)	$ (4,138)	$ (1,429)
Unrealized holding gains (losses) on available-for-sale securities, income tax effect	242	1,448	(500)
Unrealized holding (income) losses on available-for-sale securities, net of tax amount	(449)	(2,690)	(929)
Reclassification adjustment for (gains) realized in income, before tax amount	(257)	(1,158)	(2,861)
Reclassification adjustment for (gains) realized in income, income tax effect	89	406	(1,002)
Reclassification adjustment for (gains) realized in income, net of tax amount	(168)	(752)	(1,859)
Other comprehensive income (loss) on available-for-sale securities, before tax amount	(948)	(5,296)	(4,290)
Other comprehensive income (loss) on available-for-sale securities, income tax effect	331	1,854	(1,502)
Other comprehensive loss on available-for-sale securities, net of tax amount	(617)	(3,442)	(2,788)
Changes from plan amendments and actuarial gains and losses included in accumulated other comprehensive gain (loss)	36	46	(135)
Changes from plan amendments and actuarial gains and losses included in other comprehensive income, income tax effect	(12)	(16)	(47)
Changes from plan amendments and actuarial gains and losses included in other comprehensive income, net of tax amount	24	30	(88)
Amortization of net transition obligation, prior service cost and net actuarial gain included in net periodic benefit cost, before tax amount	(24)	(22)	67
Amortization of net transition obligation, prior service cost and net actuarial gain included in net periodic benefit cost, income tax effect	8	8	(23)
Amortization of net transition obligation, prior service cost and net actuarial gain included in net periodic benefit cost, net of tax amount	(16)	(14)	44
Other comprehensive (income) loss on unfunded retirement obligations, before tax amount	12	24	(68)
Other comprehensive income (loss) on unfunded retirement obligations, income tax effect	(4)	(8)	(24)
Other comprehensive income (loss) on unfunded retirement obligations, net of tax amount	8	16	(44)
Total other comprehensive loss	(936)	(5,272)	(4,358)
Total other comprehensive income (loss), income tax effect	327	1,846	(1,526)
Net other comprehensive loss	(609)	(3,426)	(2,832)
Other comprehensive income (loss) on available-for-sale securities, income tax effect	(331)	(1,854)	1,502
Unrealized holding gains on available-for-sale securities, income tax effect	(242)	(1,448)	500
Reclassification adjustment for (gains) realized in income, income tax effect	(89)	(406)	1,002
Changes from plan amendments and actuarial gains and losses included in other comprehensive income, income tax effect	12	16	47
Other comprehensive income (loss) on unfunded retirement obligations, income tax effect	4	8	24
Total other comprehensive income (loss), income tax effect	$ (327)	$ (1,846)	$ 1,526