Note 15 - Pension and Other Postretirement Benefit Plans - Schedule of Changes in Benefit Obligation, Plan Assets and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair value of plan assets at beginning of period	$ 668,481
Fair value of plan assets at end of period	740,978	$ 668,481
Pension Plan [Member]
Benefit obligation at beginning of period	720,098	644,142
Service cost	1,183	3,950	$ 1,150
Interest cost	20,176	25,101	23,505
Actuarial loss (gain)	84,663	95,078
Benefits and other payments	(47,252)	(48,173)
Benefit obligation at end of period	778,868	720,098	644,142
Fair value of plan assets at beginning of period	668,481	578,347
Actual return on plan assets	118,403	136,976
Company contributions	1,346	1,331
Benefits and other payments	(47,252)	(48,173)
Fair value of plan assets at end of period	740,978	668,481	578,347
Funded status:
Current liabilities	(2,531)	(1,687)
Noncurrent liabilities	(35,359)	(49,930)
Net obligation recognized	(37,890)	(51,617)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	256,988	255,830
Prior service credit	0	0
Net amount recognized (before tax effect)	256,988	255,830
Other Postretirement Benefits Plan [Member]
Benefit obligation at beginning of period	464,329	473,591
Service cost	0	0	0
Interest cost	12,795	18,320	18,706
Actuarial loss (gain)	(38,455)	4,761
Benefits and other payments	(24,959)	(32,343)
Benefit obligation at end of period	413,710	464,329	473,591
Fair value of plan assets at beginning of period	0	0
Actual return on plan assets	0	0
Company contributions	24,959	32,343
Benefits and other payments	(24,959)	(32,343)
Fair value of plan assets at end of period	0	0	$ 0
Funded status:
Current liabilities	(26,073)	(31,833)
Noncurrent liabilities	(387,637)	(432,496)
Net obligation recognized	(413,710)	(464,329)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	132,203	179,937
Prior service credit	(18,544)	(20,949)
Net amount recognized (before tax effect)	$ 113,659	$ 158,988