Note 2 - Investments (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Securities, Current	$ 33,800	$ 29,100
Payments to Acquire Available-for-sale Securities, Debt	55,000
Proceeds from Sale and Maturity of Marketable Securities	49,969	18,649
Available-for-sale Securities, Equity Securities, Noncurrent	7,600	5,000
Available-for-sale Equity Securities, Amortized Cost Basis	5,700	4,100
Available-for-sale Equity Securities, Acquired During Period, Cost Basis	$ 1,600	900
Other than Temporary Impairment Losses, Investments		$ 1,000