Note 8 - Retirement Plans (Details Textual) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)
Common Stock, Capital Shares Reserved for Future Issuance | shares	7,422,965	5,449,148
Capital Accumulation Plan [Member]
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 6.6	$ 5.4	$ 1.8
Defined Contribution Plan, Expected Contributions In Next Twelve Months	$ 6.8
Common Stock, Capital Shares Reserved for Future Issuance | shares	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%
Pension Plan [Member] | Assumed Plans [Member]
Number of Underfunded Pension Plans		2
Pension Plan [Member] | Gray Pension Plan [Member]
Defined Benefit Plan, Accumulated Benefit Obligation	$ 126.1	$ 115.0
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	2.0
Profit Sharing Contribution [Member]
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 4.1	$ 3.4	$ 1.6