Note 9 - Cash, Cash Equivalents, Investments and Restricted Cash - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Continuous Unrealized Loss Position, Less than 12 Months	$ 166,746	$ 65,142
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(277)	(99)
Continuous Unrealized Loss Position, 12 Months or Longer	2,582	2,861
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(138)	(159)
Unrealized Loss Position, Total	169,328	68,003
Unrealized Loss Position, Accumulated Loss, Total	(415)	(258)
Corporate Debt Securities [Member]
Continuous Unrealized Loss Position, Less than 12 Months	159,262	59,144
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(262)	(97)
Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Unrealized Loss Position, Total	159,262	59,144
Unrealized Loss Position, Accumulated Loss, Total	(262)	(97)
US Treasury and Government Short-term Debt Securities [Member]
Continuous Unrealized Loss Position, Less than 12 Months	7,484
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(15)
Continuous Unrealized Loss Position, 12 Months or Longer	0
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Unrealized Loss Position, Total	7,484
Unrealized Loss Position, Accumulated Loss, Total	(15)
US Government Agencies Short-term Debt Securities [Member]
Continuous Unrealized Loss Position, Less than 12 Months		5,998
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(2)
Continuous Unrealized Loss Position, 12 Months or Longer		0
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Unrealized Loss Position, Total		5,998
Unrealized Loss Position, Accumulated Loss, Total		(2)
Auction Rate Securities [Member]
Continuous Unrealized Loss Position, Less than 12 Months	0	0
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Continuous Unrealized Loss Position, 12 Months or Longer	2,582	2,861
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(138)	(159)
Unrealized Loss Position, Total	2,582	2,861
Unrealized Loss Position, Accumulated Loss, Total	$ (138)	$ (159)