Note 3 - Cash, Cash Equivalents, Investments and Restricted Cash - Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Amortized Cost	$ 313,546	$ 250,800
Unrealized Gains	390	9
Unrealized Losses	(261)	(1,856)
Total Fair Value	313,675	248,953
Fair Value of Investments in Unrealized Loss Position	97,393	197,952
Money Market Funds [Member]
Amortized Cost	28,100	41,135
Unrealized Gains	0	0
Unrealized Losses	0	0
Total Fair Value	28,100	41,135
Fair Value of Investments in Unrealized Loss Position	0	0
Corporate Debt Securities [Member]
Amortized Cost	260,645	172,288
Unrealized Gains	383	7
Unrealized Losses	(78)	(1,386)
Total Fair Value	260,950	170,909
Fair Value of Investments in Unrealized Loss Position	93,262	166,204
Commercial Paper [Member]
Amortized Cost	1,994
Unrealized Gains	0
Unrealized Losses	0
Total Fair Value	1,994
Fair Value of Investments in Unrealized Loss Position	0
US Government Agencies Short-term Debt Securities [Member]
Amortized Cost	19,487	32,207
Unrealized Gains	7	2
Unrealized Losses	(1)	(141)
Total Fair Value	19,493	32,068
Fair Value of Investments in Unrealized Loss Position	993	28,507
Certificates of Deposit [Member]
Amortized Cost		1,600
Unrealized Gains		0
Unrealized Losses		0
Total Fair Value		1,600
Fair Value of Investments in Unrealized Loss Position		0
Auction Rate Securities [Member]
Amortized Cost	3,320	3,570
Unrealized Gains	0	0
Unrealized Losses	(182)	(329)
Total Fair Value	3,138	3,241
Fair Value of Investments in Unrealized Loss Position	$ 3,138	$ 3,241