Note 15 - Pension and Other Postretirement Benefit Plans - Components of Net Periodic Benefit (Credit) Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Service cost	$ 1,183	$ 3,950	$ 1,150
Interest cost	20,176	25,101	23,505
Expected return on plan assets	(41,821)	(40,457)	(40,370)
Amortization of prior service credits	0	(367)	(502)
Recognized net actuarial loss	6,922	5,958	8,715
Net periodic benefit (credit) cost	(13,540)	(5,815)	(7,502)
Other Postretirement Benefits Plan [Member]
Service cost	0	0	0
Interest cost	12,795	18,320	18,706
Expected return on plan assets	0	0	0
Amortization of prior service credits	(2,405)	(2,405)	(2,405)
Recognized net actuarial loss	9,277	9,262	16,205
Net periodic benefit (credit) cost	$ 19,667	$ 25,177	$ 32,506