Note 9 - Marketable Securities - Held-to-maturity Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Short-term marketable securities	$ 37,918	$ 30,002	$ 35,010
Long-term marketable securities	10,000	36,098	46,093
Total marketable securities	47,918	66,100	81,103
US Treasury and Government [Member]
Short-term marketable securities	37,918	24,996	30,000
Long-term marketable securities	10,000	36,098	46,093
Corporate Bond Securities [Member]
Short-term marketable securities	$ 0	$ 5,006	$ 5,010