Note 4 - Investments in Debt Securities - Amortized Cost and Fair Value of Investments in Debt Securities (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Investment in debt securities, amortized cost	$ 17,237	$ 27,372
Investment in debt securities, fair value	17,263	27,488
Investment in debt securities, unrealized gains (losses)	26	116
Certificates of Deposit [Member]
Investment in debt securities, amortized cost	750	1,150
Investment in debt securities, fair value	750	1,150
Investment in debt securities, unrealized gains (losses)	0	0
US Treasury and Government [Member]
Investment in debt securities, amortized cost	4,550	0
Investment in debt securities, fair value	4,549	0
Investment in debt securities, unrealized gains (losses)	(1)	0
Corporate Debt Securities [Member]
Investment in debt securities, amortized cost	11,937	26,222
Investment in debt securities, fair value	11,964	26,338
Investment in debt securities, unrealized gains (losses)	$ 27	$ 116