Note 3 - Supplemental Cash Flow Disclosure - Supplemental Disclosures of Cash Flow Information and Non-cash Investing and Financing Activities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash paid for:
Interest, net of interest capitalized	$ 726	$ 637	$ 717
Income taxes	30,157	49,830	29,144
Non-cash investing and financing activities:
Unrealized holding gains (losses) on marketable securities, net of tax	$ (2,536)	$ 15,204	$ 6,127