Note 9 - Employee Benefit Plans (Details Textual) - USD ($)	1 Months Ended	2 Months Ended	12 Months Ended
	Jan. 31, 2021	Feb. 18, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets			6.45%	6.37%
Defined Benefit Plan Assumed Long Term Rate Basis Spread			1.00%
Defined Benefit Plan, Plan Assets, Contributions by Employer			$ 23,731,000	$ 4,095,000
Defined Contribution Plan, Employer Matching Contribution, Percent of Match			55.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay			5.00%
Defined Contribution Plan, Employer Discretionary Contribution Amount			$ 10,000	10,000	$ 4,000
Capital Accumulation 401(K) Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer			$ 4,600,000	3,900,000	3,700,000
Defined Contribution Plan, Employer Matching Contribution, Percent of Match			100.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay			6.00%
Minimum [Member]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent			2.00%
Minimum [Member] | Capital Accumulation 401(K) Plan [Member]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent			1.00%
Maximum [Member]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent			50.00%
Maximum [Member] | Capital Accumulation 401(K) Plan [Member]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent			50.00%
Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer			$ 16,000,000.0
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year			6,000,000.0
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year			800,000
Defined Benefit Plan, Equity Securities, Common Stock [Member]
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Amount			17,200,000	14,700,000
Nonoperating Income (Expense) [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Except Service cost			$ 2,900,000	$ 5,000,000.0	$ 2,800,000
Subsequent Event [Member]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets		6.40%
Subsequent Event [Member] | Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 16,800,000