Note 9 - Cash, Cash Equivalents, Investments and Restricted Cash - Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Amortized Cost	$ 381,221	$ 313,546
Unrealized Gains	3,964	390
Unrealized Losses	(227)	(261)
Total Fair Value	384,958	313,675
Money Market Funds [Member]
Amortized Cost	26,086	28,100
Unrealized Gains	0	0
Unrealized Losses	0	0
Total Fair Value	26,086	28,100
Corporate Debt Securities [Member]
Amortized Cost	329,755	260,645
Unrealized Gains	3,930	383
Unrealized Losses	(63)	(78)
Total Fair Value	333,622	260,950
Commercial Paper [Member]
Amortized Cost	7,936	1,994
Unrealized Gains	32	0
Unrealized Losses	0	0
Total Fair Value	7,968	1,994
US Government Agencies Short-term Debt Securities [Member]
Amortized Cost	14,249	19,487
Unrealized Gains	2	7
Unrealized Losses	(1)	(1)
Total Fair Value	14,250	19,493
Auction Rate Securities [Member]
Amortized Cost	3,195	3,320
Unrealized Gains	0	0
Unrealized Losses	(163)	(182)
Total Fair Value	$ 3,032	$ 3,138