Note 11 - Postretirement Medical Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Curtailment gain recognized			$ (417)
Postretirement Health Coverage [Member]
Service cost	$ 141	$ 137		$ 281	$ 283
Interest cost	152	164		305	332
Curtailment gain recognized	0				(351)
Recognized prior service cost	0				8
Net periodic benefit cost	$ 293	$ 301		$ 586	$ 272