Note 10 - Fair Value Measurements - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Total Fair Value	$ 443,899	$ 297,365
Assets, fair value	472,151	324,289
Fair Value, Inputs, Level 1 [Member]
Assets, fair value	48,876	61,259
Fair Value, Inputs, Level 2 [Member]
Assets, fair value	420,455	260,169
Fair Value, Inputs, Level 3 [Member]
Assets, fair value	2,820	2,861
Money Market Funds [Member]
Total Fair Value	20,624	34,335
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	20,624	34,335
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	0	0
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	0	0
Certificates of Deposit [Member]
Total Fair Value	106,763
Certificates of Deposit [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	0
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	106,763
Certificates of Deposit [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	0
Corporate Debt Securities [Member]
Total Fair Value	306,201	249,671
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	0	0
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	306,201	249,671
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	0	0
Commercial Paper [Member]
Total Fair Value		2,999
Commercial Paper [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value		0
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value		2,999
Commercial Paper [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value		0
US Government Agencies Short-term Debt Securities [Member]
Total Fair Value	7,491	7,499
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	0	0
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	7,491	7,499
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	0	0
Auction Rate Securities [Member]
Total Fair Value	2,820	2,861
Auction Rate Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	0	0
Auction Rate Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	0	0
Auction Rate Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	2,820	2,861
Mutual Fund [Member]
Total Fair Value	28,252	26,924
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	28,252	26,924
Mutual Fund [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	0	0
Mutual Fund [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	$ 0	$ 0