Note 9 - Cash, Cash Equivalents, Investments and Restricted Cash - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Continuous Unrealized Loss Position, Less than 12 Months	$ 217,538	$ 83,119
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,019)	(64)
Continuous Unrealized Loss Position, 12 Months or Longer	3,057	14,274
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(163)	(197)
Unrealized Loss Position, Total	220,595	97,393
Unrealized Loss Position, Accumulated Loss, Total	(2,182)	(261)
Corporate Debt Securities [Member]
Continuous Unrealized Loss Position, Less than 12 Months	215,069	82,126
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,017)	(63)
Continuous Unrealized Loss Position, 12 Months or Longer	0	11,136
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(15)
Unrealized Loss Position, Total	215,069	93,262
Unrealized Loss Position, Accumulated Loss, Total	(2,017)	(78)
Commercial Paper [Member]
Continuous Unrealized Loss Position, Less than 12 Months	2,469
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	2,469
Unrealized Loss Position, Accumulated Loss, Total	(2)
US Government Agencies Short-term Debt Securities [Member]
Continuous Unrealized Loss Position, Less than 12 Months		993
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(1)
Continuous Unrealized Loss Position, 12 Months or Longer		0
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Unrealized Loss Position, Total		993
Unrealized Loss Position, Accumulated Loss, Total		(1)
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	3,057	3,138
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(163)	(182)
Unrealized Loss Position, Total	3,057	3,138
Unrealized Loss Position, Accumulated Loss, Total	$ (163)	$ (182)