Note 6 - Accrued Retirement Benefits - Changes in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Change in benefit obligations:
Benefit obligations	$ 51,306	$ 56,453
Interest cost	2,096	1,974
Actuarial loss (gain)	4,706	(3,096)
Benefits paid	(3,980)	(4,025)
Benefit obligations	54,128	51,306
Change in plan assets:
Fair value of plan assets	41,290	48,442
Actual return on plan assets	6,814	(3,114)
Employer reimbursement for retirement benefits		(118)
Employer contributions	160	105
Benefits paid	(3,980)	(4,025)
Fair value	44,284	41,290
Funded status	(9,844)	(10,016)
Accumulated benefit obligations	$ 54,128	$ 51,306