Note 9 - Cash, Cash Equivalents, Investments and Restricted Cash - Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Amortized Cost	$ 349,976	$ 313,546
Unrealized Gains	603	390
Unrealized Losses	(2,182)	(261)
Total Fair Value	348,397	313,675
Money Market Funds [Member]
Amortized Cost	10,954	28,100
Unrealized Gains	0	0
Unrealized Losses	0	0
Total Fair Value	10,954	28,100
Corporate Debt Securities [Member]
Amortized Cost	310,367	260,645
Unrealized Gains	559	383
Unrealized Losses	(2,017)	(78)
Total Fair Value	308,909	260,950
Commercial Paper [Member]
Amortized Cost	4,942	1,994
Unrealized Gains	3	0
Unrealized Losses	(2)	0
Total Fair Value	4,943	1,994
US Government Agencies Short-term Debt Securities [Member]
Amortized Cost	20,493	19,487
Unrealized Gains	41	7
Unrealized Losses	0	(1)
Total Fair Value	20,534	19,493
Auction Rate Securities [Member]
Amortized Cost	3,220	3,320
Unrealized Gains	0	0
Unrealized Losses	(163)	(182)
Total Fair Value	$ 3,057	$ 3,138