Note 9 - Cash, Cash Equivalents, Investments and Restricted Cash - Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost	$ 531,242	$ 295,444
Unrealized Gains	1,123	2,179
Unrealized Losses	(448)	(258)
Total Fair Value	531,917	297,365
Money Market Funds [Member]
Amortized Cost	13,469	34,335
Unrealized Gains	0	0
Unrealized Losses	0	0
Total Fair Value	13,469	34,335
Certificates of Deposit [Member]
Amortized Cost	139,361
Unrealized Gains	0
Unrealized Losses	0
Total Fair Value	139,361
Corporate Debt Securities [Member]
Amortized Cost	368,298	247,591
Unrealized Gains	1,122	2,177
Unrealized Losses	(326)	(97)
Total Fair Value	369,094	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	7,494	7,499
Unrealized Gains	1	2
Unrealized Losses	(3)	(2)
Total Fair Value	7,492	7,499
Auction Rate Securities [Member]
Amortized Cost	2,620	3,020
Unrealized Gains	0	0
Unrealized Losses	(119)	(159)
Total Fair Value	$ 2,501	$ 2,861