Note 6 - Accrued Retirement Benefits - Fair Values of Pension Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair value of pension plan assets	$ 44,284	$ 41,290	$ 48,442
Fair Value, Inputs, Level 1 [Member]
Fair value of pension plan assets
Fair Value, Inputs, Level 2 [Member]
Fair value of pension plan assets	44,284	41,290
Equity Funds [Member]
Fair value of pension plan assets	12,714	10,665
Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value of pension plan assets
Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value of pension plan assets	12,714	10,665
Fixed Income Funds [Member]
Fair value of pension plan assets	30,548	29,635
Fixed Income Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value of pension plan assets
Fixed Income Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value of pension plan assets	30,548	29,635
Money Market Funds [Member]
Fair value of pension plan assets	1,022	990
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value of pension plan assets
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value of pension plan assets	$ 1,022	$ 990