Note 3 - Cash, Cash Equivalents, Investments and Restricted Cash (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 200	$ 1,600
Other-than-temporary Impairment Loss, Debt Securities, Available-for-sale, Total	$ 0
Auction Rate Securities Maturities	27 years
Redemption, Auction Rate Securities to Date	$ 40,000
Percentage of Auction Rate Securities Redeemed at Par Value	92.00%
Auction Rate Securities [Member]
Redemption Auction Rate Securities	$ 300	2,000	$ 0
Other than Temporary Impairment Losses, Investments, Total	$ 200	$ 300