Note 6 - Accrued Retirement Benefits - Changes in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Change in benefit obligations:
Benefit obligations	$ 54,128	$ 51,306
Interest cost	1,633	2,096
Actuarial loss	3,588	4,706
Benefits paid	(4,694)	(3,980)
Benefit obligations	54,655	54,128
Change in plan assets:
Fair value of plan assets	44,284	41,290
Actual return on plan assets	3,869	6,814
Employer contributions	128	160
Benefits paid	(4,694)	(3,980)
Fair value	43,587	44,284
Funded status	(11,068)	(9,844)
Accumulated benefit obligations	$ 54,655	$ 54,128
Discount rate
Expected long-term return on plan assets	4.50%	4.75%
Minimum [Member]
Change in plan assets:
Discount rate	2.28%	3.10%
Maximum [Member]
Change in plan assets:
Discount rate	2.35%	3.14%