Note 6 - Accrued Retirement Benefits - Fair Values of Pension Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair value of pension plan assets	$ 43,587	$ 44,284	$ 41,290
Equity Funds [Member]
Fair value of pension plan assets	10,558	12,714
Fixed Income Funds [Member]
Fair value of pension plan assets	32,066	30,548
Money Market Funds [Member]
Fair value of pension plan assets	963	1,022
Fair Value, Inputs, Level 1 [Member]
Fair value of pension plan assets
Fair Value, Inputs, Level 1 [Member] | Equity Funds [Member]
Fair value of pension plan assets
Fair Value, Inputs, Level 1 [Member] | Fixed Income Funds [Member]
Fair value of pension plan assets
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Fair value of pension plan assets
Fair Value, Inputs, Level 2 [Member]
Fair value of pension plan assets	40,327	44,284
Fair Value, Inputs, Level 2 [Member] | Equity Funds [Member]
Fair value of pension plan assets	9,406	12,714
Fair Value, Inputs, Level 2 [Member] | Fixed Income Funds [Member]
Fair value of pension plan assets	29,958	30,548
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Fair value of pension plan assets	963	$ 1,022
Fair Value, Inputs, Level 3 [Member]
Fair value of pension plan assets	3,260
Fair Value, Inputs, Level 3 [Member] | Equity Funds [Member]
Fair value of pension plan assets	1,152
Fair Value, Inputs, Level 3 [Member] | Fixed Income Funds [Member]
Fair value of pension plan assets	2,108
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Fair value of pension plan assets