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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Employee Pension Plan [Member]
Interest cost	$ 195	$ 216	$ 390	$ 432
Amortization of unrecognized loss	156	174	311	348
Expected return on plan assets	(363)	(348)	(726)	(696)
Net employee pension ( benefit) expense	(12)	42	(25)	84
Directors' Plan [Member]
Interest cost	20	23	40	46
Amortization of unrecognized loss	(23)	(23)	(46)	(46)
Net employee pension ( benefit) expense	11	20	22	40
Service cost	11	10	22	20
Amortization of past service liability	3	10	6	20
Other Postretirement Benefit Plans [Member]
Interest cost	77	76	154	152
Net employee pension ( benefit) expense	153	134	305	268
Service cost	88	79	176	158
Amortization of past service liability	$ (12)	$ (21)	$ (25)	$ (42)