Note 6 - Retirement Plans (Details Textual) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
Defined Contribution Plan, Employer Discretionary Contribution Amount		$ 0
Capital Accumulation Plan [Member]
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 1,600	3,500
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	3,200	3,200
Pension Plan [Member] | Gray Pension Plan [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	$ 2,000	$ 2,000