Note 9 - Other Comprehensive Income (Details Textual) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017		Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2016
Other Comprehensive Income (Loss), Remeasurement of Deferred Tax Assets and Liabilities		$ 506
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%		35.00%	21.00%	35.00%	35.00%
Stockholders' Equity Attributable to Parent, Ending Balance	$ 352,980		$ 334,255	$ 352,980	$ 334,255	$ 333,644				$ 313,872
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent, Ending Balance	(8,906)		(281)	(8,906)	(281)	(1,781)	[1]	$ (7,274)	$ (317)	(1,211)
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | Previously Reported [Member]
Stockholders' Equity Attributable to Parent, Ending Balance						(2,278)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent, Ending Balance	663		22	663	22	193	[1]	623	(21)	(16)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Previously Reported [Member]
Stockholders' Equity Attributable to Parent, Ending Balance						234
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent, Ending Balance	$ (393)		$ (272)	$ (393)	$ (272)	(342)	[1]	$ (393)	$ (272)	$ (272)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Previously Reported [Member]
Stockholders' Equity Attributable to Parent, Ending Balance						$ (392)

[1] December 31, 2017 AOCI component balances reflect a correction of incorrectly reported year-end balances in Note 12 of the 2017 Form 10-K, which were presented as $(2,278), $234, and $(392) for securities available-for-sale, cash flow hedges, and minimum pension liability, respectively.