Note 15 - Pension and Other Postretirement Benefit Plans - Schedule of Fair Value of Plan Assets (Details) - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019
Fair value of plan assets		$ 740,978	$ 668,481
Fair Value, Inputs, Level 1 [Member]
Fair value of plan assets		100	97
Fair Value, Inputs, Level 2 [Member]
Fair value of plan assets		0	0
Fair Value, Inputs, Level 3 [Member]
Fair value of plan assets		0	0
Defined Benefit Plan, Cash and Cash Equivalents [Member]
Fair value of plan assets		100	97
Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value of plan assets		100	97
Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value of plan assets		0	0
Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value of plan assets		0	0
Private Equity Funds [Member]
Fair value of plan assets	[1]	740,878	668,384
Private Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value of plan assets	[1]	0	0
Private Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value of plan assets	[1]	0	0
Private Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value of plan assets	[1]	$ 0	$ 0

[1] Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy but are included in the total.