Note 4 - Employee Benefit Plans (Details Textual) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
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		$ 0.8		$ 0.8
Other Nonoperating Income (Expense) [Member]
Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component		$ 0.2	$ 0.7	$ 0.3	$ 1.4