Note 6 - Accrued Retirement Benefits - Fair Values of Pension Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair value of pension plan assets	$ 41,290	$ 48,442	$ 47,176
Fair Value, Inputs, Level 1 [Member]
Fair value of pension plan assets
Fair Value, Inputs, Level 2 [Member]
Fair value of pension plan assets	41,290	48,442
Equity Funds [Member]
Fair value of pension plan assets	10,665	14,772
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	10,665	14,772
Fixed Income Funds [Member]
Fair value of pension plan assets	29,635	32,581
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	29,635	32,581
Money Market Funds [Member]
Fair value of pension plan assets	990	1,089
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	$ 990	$ 1,089