Note 13 - Postretirement Medical Benefits (Details Textual) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Curtailment	$ 417
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Prior Service Costs Recognized Due to Curtailment	66
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Curtailment, Net of Prior Service Costs Recognized	$ 351	$ 351	$ 0
Defined Benefit Plan, Plan Assets, Contributions by Employer		354	$ 309
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		$ 459
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year		6.50%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Year Two		6.00%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Year Three		5.50%
Defined Benefit Plan, Change in Health Care Cost Trend Rate Assumed		(0.25%)
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Five Fiscal Years Thereafter		4.75%
Defined Benefit Plan, Dental Care Cost Trend Rate Assumed, Next Fiscal Year and Thereafter		5.00%
Plan 2 [Member]
Employee Service Requirement		20 years
Employee Age Requirement		55 years
Contributions Plan 2		50.00%
Plan 1 [Member]
Company Contributions Plan 1		100.00%