Note 15 - Pension and Other Postretirement Benefit Plans (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost Assumed Time Horizon (Year)	20 years
Defined Benefit Plan, Plan Assets, Amount, Ending Balance	$ 740,978	$ 668,481
Defined Benefit Plan, Expected Future Benefit Payment, Year One	10,580
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	0	0
Defined Benefit Plan, Plan Assets, Amount, Ending Balance	0	0	$ 0
Defined Benefit Plan, Expected Future Benefit Payment, Year One	26,073
Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Amount, Ending Balance	740,978	$ 668,481	$ 578,347
Defined Benefit Plan, Expected Future Benefit Payment, Year One	44,391
Pension Plan [Member] | Nonqualified Plan [Member]
Defined Benefit Plan, Expected Future Benefit Payment, Year One	$ 2,531
United States Equity Securities [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	21.00%
Non U.S. Equity Securities [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	13.00%
Global Equity Securities [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	6.00%
Fixed Equity Securities [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	60.00%