Note 6 - Business Segments and Sales of Products - Sales by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Sales by geographical area	$ 144,563	$ 133,472	$ 283,657	$ 280,108
CANADA
Sales by geographical area	94,524	83,215	183,192	183,434
KOREA, REPUBLIC OF
Sales by geographical area	41,537	18,972	74,240	47,943
JAPAN
Sales by geographical area	3,897	11,804	17,670	24,094
CHINA
Sales by geographical area	198	15,401	67	15,352
UNITED STATES
Sales by geographical area	$ 4,407	$ 4,080	$ 8,488	$ 9,285