Note 13 - Postretirement Medical Benefits - Incremental Effects From a One Percent Change in Assumed Health Care Cost (Details) - Postretirement Health Coverage [Member] $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Effect on total of service and interest cost components of net periodic postretirement health care benefit cost, 1% Increase	$ 295
Effect on total of service and interest cost components of net periodic postretirement health care benefit cost, 1% Decrease	(247)
Effect on accumulated postretirement benefit obligation, 1% Increase	3,530
Effect on accumulated postretirement benefit obligation, 1% Decrease	$ (2,733)