Note 8 - 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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Employee Pension Plan [Member]
Interest cost	$ 216	$ 226	$ 648	$ 678
Amortization of unrecognized loss	174	201	523	604
Expected return on plan assets	(348)	(348)	(1,044)	(1,044)
Net expense	42	79	127	238
Directors' Plan [Member]
Interest cost	23	24	69	72
Amortization of unrecognized loss	(23)	(21)	(69)	(65)
Net expense	20	23	60	70
Service cost	10	11	30	33
Amortization of past service liability	10	9	30	30
Other Postretirement Benefit Plans [Member]
Interest cost	76	80	228	240
Amortization of unrecognized loss		12		36
Net expense	134	160	401	482
Service cost	79	90	237	270
Amortization of past service liability	$ (21)	$ (22)	$ (64)	$ (64)