Note 3 - Investments (Details Textual) - USD ($) $ in Thousands	6 Months Ended	9 Months Ended
	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Securities, Available-for-sale, Current, Total		$ 0		$ 33,758
Payments to Acquire Debt Securities, Available-for-sale		31,200	$ 35,300
Proceeds from Sale and Maturity of Marketable Securities, Total		64,895	31,169
Equity Securities, FV-NI		7,200		7,600
Equity Securities, FV-NI, Cost		7,900		$ 5,700
Available-for-sale Equity Securities, Acquired During Period, Cost Basis		800	1,600
Equity Securities, FV-NI, Unrealized Gain (Loss), Total	$ 100	$ (2,500)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax			$ 1,500