Note 9 - Retirement Plans (Details Textual) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common Stock, Capital Shares Reserved for Future Issuance	7,078,916	7,422,965
Pension Plan [Member] | Gray Pension Plan [Member]
Defined Benefit Plan, Accumulated Benefit Obligation	$ 116.7	$ 126.1
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	2.5
Capital Accumulation Plan [Member]
Defined Contribution Plan, Employer Discretionary Contribution Amount	6.4	6.6	$ 5.4
Defined Contribution Plan, Expected Contributions In Next Twelve Months	$ 12.5
Common Stock, Capital Shares Reserved for Future Issuance	1,587,719
Capital Accumulation Plan [Member] | First 3% of Each Employee's Salary Deferral [Member]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	100.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	3.00%
Capital Accumulation Plan [Member] | Next 2% of Each Employee's Salary Deferral [Member]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	50.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	2.00%
Profit Sharing Contribution [Member]
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 4.3	$ 4.1	$ 3.4