Note 12 - Pension and Other Postretirement Benefits - The Components of the Net Pension Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Retirement Plan [Member]
Interest cost	$ 781	$ 864	$ 902
Amortization of unrecognized (gain) loss	621	697	809
Expected return on plan assets	(1,452)	(1,392)	(1,394)
Net pension (benefit) expense	(50)	169	317
Current year actuarial (gain) loss	(2,307)	(1,192)	275
Amortization of actuarial gain (loss)	(621)	(697)	(809)
Total recognized in other comprehensive income	(2,928)	(1,889)	(534)
Total recognized in net pension cost (benefit) and other comprehensive loss	(2,978)	(1,720)	(217)
Amortization of actuarial gain (loss)	621	697	809
Other Postretirement Benefits Plan [Member]
Interest cost	307	305	320
Amortization of unrecognized (gain) loss	33		47
Net pension (benefit) expense	605	536	641
Current year actuarial (gain) loss	(1,155)	587	(613)
Amortization of actuarial gain (loss)	(33)		(47)
Total recognized in other comprehensive income	(1,103)	672	(575)
Total recognized in net pension cost (benefit) and other comprehensive loss	(498)	1,208	66
Service cost	350	316	359
Amortization of past service credit	(85)	(85)	(85)
Amortization of prior service credit	85	85	85
Amortization of actuarial gain (loss)	33		47
Directors' Plan [Member]
Interest cost	78	89	97
Amortization of unrecognized (gain) loss	(91)	(92)	(86)
Net pension (benefit) expense	41	79	93
Current year actuarial (gain) loss	(184)	(24)	(63)
Amortization of actuarial gain (loss)	(91)	(92)	(86)
Total recognized in other comprehensive income	(105)	28	(17)
Total recognized in net pension cost (benefit) and other comprehensive loss	(64)	107	76
Service cost	42	42	42
Amortization of past service credit	12	40	40
Amortization of prior service credit	(12)	(40)	(40)
Amortization of actuarial gain (loss)	$ 91	$ 92	$ 86