Note 1 - Basis of Presentation (Details Textual) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Number of Operating Segments				1
Interchange Income (Expense), Net	$ 1,470		$ 1,300	$ 2,800	$ 2,530
Interchange Income	1,950		1,720	3,720	3,330
Interchange Expense	$ 487		$ 421	$ 915	$ 799
Accounting Standards Update 2017-07 [Member] | Reclassification of Non-service Components of Net Period Benefit Costs From Salaries and Employee Benefits to Other Expense [Member] | Three Months Ended June 30, 2017 [Member]
Prior Period Reclassification Adjustment		$ 158
Accounting Standards Update 2017-07 [Member] | Reclassification of Non-service Components of Net Period Benefit Costs From Salaries and Employee Benefits to Other Expense [Member] | Six Months Ended June 30, 2017 [Member]
Prior Period Reclassification Adjustment		$ 315