Note 9 - Cash, Cash Equivalents, Investments and Restricted Cash - Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost	$ 412,848	$ 313,546
Unrealized Gains	3,608	390
Unrealized Losses	(269)	(261)
Total Fair Value	416,187	313,675
Money Market Funds [Member]
Amortized Cost	41,195	28,100
Unrealized Gains	0	0
Unrealized Losses	0	0
Total Fair Value	41,195	28,100
Corporate Debt Securities [Member]
Amortized Cost	345,865	260,645
Unrealized Gains	3,590	383
Unrealized Losses	(113)	(78)
Total Fair Value	349,342	260,950
Commercial Paper [Member]
Amortized Cost	7,970	1,994
Unrealized Gains	15	0
Unrealized Losses	0	0
Total Fair Value	7,985	1,994
US Government Agencies Short-term Debt Securities [Member]
Amortized Cost	14,748	19,487
Unrealized Gains	3	7
Unrealized Losses	(2)	(1)
Total Fair Value	14,749	19,493
Auction Rate Securities [Member]
Amortized Cost	3,070	3,320
Unrealized Gains	0	0
Unrealized Losses	(154)	(182)
Total Fair Value	$ 2,916	$ 3,138