Note 7 - Employee Benefit Plans (Details Textual) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 30, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 0.4
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year		$ 10.0		$ 10.0
Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year		4.8		4.8
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	$ 1.4	$ 2.5