Note 2 - Investments (Details Textual) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Debt Securities, Available-for-sale, Current, Total	$ 34,358		$ 33,758
Payments to Acquire Debt Securities, Available-for-sale	31,200	$ 11,100
Proceeds from Sale and Maturity of Marketable Securities, Total	30,501	3,634
Equity Securities, FV-NI	7,700		7,600
Equity Securities, FV-NI, Cost	5,600		5,700
Equity Securities, Accumulated Unrealized Gain (Loss)	2,100		$ 1,900
Equity Securities, FV-NI, Unrealized Gain (Loss), Total	$ 300	(300)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax		$ (300)