Note 9 - Cash, Cash Equivalents, Investments and Restricted Cash - Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost	$ 464,811	$ 295,444
Unrealized Gains	1,388	2,179
Unrealized Losses	(415)	(258)
Total Fair Value	465,784	297,365
Money Market Funds [Member]
Amortized Cost	13,124	34,335
Unrealized Gains	0	0
Unrealized Losses	0	0
Total Fair Value	13,124	34,335
Certificates of Deposit [Member]
Amortized Cost	108,395
Unrealized Gains	0
Unrealized Losses	0
Total Fair Value	108,395
Corporate Debt Securities [Member]
Amortized Cost	333,073	247,591
Unrealized Gains	1,388	2,177
Unrealized Losses	(262)	(97)
Total Fair Value	334,199	249,671
Commercial Paper [Member]
Amortized Cost		2,999
Unrealized Gains		0
Unrealized Losses		0
Total Fair Value		2,999
US Government Agencies Short-term Debt Securities [Member]
Amortized Cost	7,499	7,499
Unrealized Gains	0	2
Unrealized Losses	(15)	(2)
Total Fair Value	7,484	7,499
Auction Rate Securities [Member]
Amortized Cost	2,720	3,020
Unrealized Gains	0	0
Unrealized Losses	(138)	(159)
Total Fair Value	$ 2,582	$ 2,861