Note 6 - Retirement Plans (Details Textual) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 0
Capital Accumulation Plan [Member]
Defined Contribution Plan, Employer Discretionary Contribution Amount	1,900
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	5,400
Profit Sharing Contribution [Member]
Defined Contribution Plan, Employer Discretionary Contribution Amount	4,100
Pension Plan [Member] | Gray Pension Plan [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	$ 2,000