Note 10 - Fair Value Measurements - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Total Fair Value	$ 465,784	$ 297,365
Assets, fair value	495,580	324,289
Fair Value, Inputs, Level 1 [Member]
Assets, fair value	42,920	61,259
Fair Value, Inputs, Level 2 [Member]
Assets, fair value	450,078	260,169
Fair Value, Inputs, Level 3 [Member]
Assets, fair value	2,582	2,861
Money Market Funds [Member]
Total Fair Value	13,124	34,335
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	13,124	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	108,395
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	108,395
Certificates of Deposit [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	0
Corporate Debt Securities [Member]
Total Fair Value	334,199	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	334,199	249,671
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	0	0
US Government Agencies Short-term Debt Securities [Member]
Total Fair Value	7,484	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	7,484	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	2,582	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	2,582	2,861
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
Mutual Fund [Member]
Total Fair Value	29,796	26,924
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	29,796	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