Note 10 - Postretirement Medical Benefits (Details Textual) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2017	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Curtailment	$ 351	$ 351	$ 0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Prior Service Costs Recognized Due to Curtailment	$ 67
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year		468
Payment for Other Postretirement Benefits		$ 172
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%