Note 7 - Employee Benefit Plans (Details Textual) - USD ($) $ in Millions	1 Months Ended			3 Months Ended		9 Months Ended
	Oct. 31, 2020	Aug. 31, 2020	Apr. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer		$ 12.4	$ 0.4
Pension Plan [Member] | Subsequent Event [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 6.0
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year				$ 0.6		$ 0.6
Other Nonoperating Income (Expense) [Member]
Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component				$ 0.7	$ 1.2	$ 2.2	$ 3.7