Note 11 - Postretirement Medical Benefits (Details Textual) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
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	$ 0	$ 351
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	459
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 96
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%