Note 9 - 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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Employee Pension Plan [Member]
Interest cost	$ 195	$ 216	$ 585	$ 648
Amortization of unrecognized loss	155	174	465	523
Expected return on plan assets	(363)	(348)	(1,089)	(1,044)
Net employee pension ( benefit) expense	(13)	42	(39)	127
Directors' Plan [Member]
Interest cost	19	23	60	69
Amortization of unrecognized loss	(23)	(23)	(69)	(69)
Net employee pension ( benefit) expense	10	20	33	60
Service cost	11	10	33	30
Amortization of past service liability	3	10	9	30
Other Postretirement Benefit Plans [Member]
Interest cost	77	76	231	228
Net employee pension ( benefit) expense	152	134	458	401
Service cost	88	79	264	237
Amortization of past service liability	$ (13)	$ (21)	$ (37)	$ (64)