Note 10 - Pension and Other Postretirement Benefit Plans - Components of Net Expense for Pension, Outside, and Other Postretirement Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Employee Pension Plan [Member]
Net employee pension benefit	$ (6)	$ (13)
Employee Pension Plan [Member]
Interest cost	199	195
Amortization of unrecognized loss	67	155
Expected return on plan assets	(272)	(363)
Directors' Plan [Member]
Interest cost	21	20
Amortization of unrecognized loss	(35)	(23)
Net employee pension benefit	(4)	11
Service cost	10	11
Amortization of past service liability		3
Other Postretirement Benefit Plans [Member]
Interest cost	85	77
Net employee pension benefit	133	152
Service cost	70	88
Amortization of past service liability	$ (22)	$ (13)