Note 5 - Employee Benefit Plans (Details Textual) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jan. 01, 2021
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 16.8
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year		$ 5.5		$ 5.5		$ 0.8
Other Nonoperating Income (Expense) [Member]
Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component		$ 0.2	$ 0.7	$ 0.5	$ 2.2