Note 3 - Cash, Cash Equivalents, Investments and Restricted Cash - Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost	$ 295,444	$ 313,546
Unrealized Gains	2,179	390
Unrealized Losses	(258)	(261)
Total Fair Value	297,365	313,675
Money Market Funds [Member]
Amortized Cost	34,335	28,100
Unrealized Gains	0	0
Unrealized Losses	0	0
Total Fair Value	34,335	28,100
Corporate Debt Securities [Member]
Amortized Cost	247,591	260,645
Unrealized Gains	2,177	383
Unrealized Losses	(97)	(78)
Total Fair Value	249,671	260,950
Commercial Paper [Member]
Amortized Cost	2,999	1,994
Unrealized Gains	0	0
Unrealized Losses	0	0
Total Fair Value	2,999	1,994
US Government Agencies Short-term Debt Securities [Member]
Amortized Cost	7,499	19,487
Unrealized Gains	2	7
Unrealized Losses	(2)	(1)
Total Fair Value	7,499	19,493
Auction Rate Securities [Member]
Amortized Cost	3,020	3,320
Unrealized Gains	0	0
Unrealized Losses	(159)	(182)
Total Fair Value	$ 2,861	$ 3,138