Note 13 - Retirement Plans (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year	$ 593
Defined Benefit Plan, Fair Value of Plan Assets	12,534
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	615	$ 662	$ 936
Postretirement Health Coverage [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(330)
Defined Benefit Plan, Fair Value of Plan Assets	59
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	15	12
401K [Member]
Defined Contribution Plan, Employer Discretionary Contribution Amount	212	186	187
Defined Benefit Plan, Plan Assets, Contributions by Employer	721	798
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(3,264)	(4,389)
Defined Benefit Plan, Fair Value of Plan Assets	12,534	11,393	11,202
Defined Benefit ACS Retirement Plan [Member]
Defined Benefit Plan, Plan Assets, Contributions by Employer	721	798	$ 779
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	288
Defined Benefit ACS Retirement Plan [Member] | Other Noncurrent Liabilities [Member]
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position	$ 3,264	$ 4,389