Note 7 - Employee Benefit Plans (Details Textual) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Apr. 30, 2020	Mar. 31, 2020	Mar. 31, 2019
Pension Plan [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year		$ 5.8
Pension Plan [Member] | Subsequent Event [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 0.4
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year		0.6
Other Nonoperating Income (Expense) [Member]
Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component		$ 0.7	$ 1.2