Note 2 - Investments (Details Textual) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Debt Securities, Available-for-sale, Current, Total	$ 5,556		$ 33,758
Payments to Acquire Debt Securities, Available-for-sale	31,200	$ 23,300
Proceeds from Sale and Maturity of Marketable Securities, Total	59,336	14,356
Equity Securities, FV-NI	7,400		7,600
Equity Securities, FV-NI, Cost	6,000		$ 5,700
Equity Securities, FV-NI, Unrealized Gain (Loss), Total	$ (300)	100
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax		$ 600