Note 9 - Cash, Cash Equivalents, Investments and Restricted Cash - Continuous Unrealized Loss Position (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Continuous Unrealized Loss Position, Less than 12 Months	$ 65,918,000	$ 83,119,000
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(115,000)	(64,000)
Continuous Unrealized Loss Position, 12 Months or Longer	2,916,000	14,274,000
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(154,000)	(197,000)
Unrealized Loss Position, Total	68,834,000	97,393,000
Unrealized Loss Position, Accumulated Loss, Total	(269,000)	(261,000)
Corporate Debt Securities [Member]
Continuous Unrealized Loss Position, Less than 12 Months	60,921,000	82,126,000
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(113,000)	(63,000)
Continuous Unrealized Loss Position, 12 Months or Longer	0	11,136,000
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(15,000)
Unrealized Loss Position, Total	60,921,000	93,262,000
Unrealized Loss Position, Accumulated Loss, Total	(113,000)	(78,000)
US Treasury and Government Short-term Debt Securities [Member]
Continuous Unrealized Loss Position, Less than 12 Months	4,997,000
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,000)
Continuous Unrealized Loss Position, 12 Months or Longer	0
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Unrealized Loss Position, Total	4,997,000
Unrealized Loss Position, Accumulated Loss, Total	(2,000)
US Government Agencies Short-term Debt Securities [Member]
Continuous Unrealized Loss Position, Less than 12 Months		993,000
Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(1,000)
Continuous Unrealized Loss Position, 12 Months or Longer		0
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Unrealized Loss Position, Total		993,000
Unrealized Loss Position, Accumulated Loss, Total		(1,000)
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	2,916,000	3,138,000
Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(154,000)	(182,000)
Unrealized Loss Position, Total	2,916,000	3,138,000
Unrealized Loss Position, Accumulated Loss, Total	$ (154,000)	$ (182,000)