Note 10 - Fair Value Measurements - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Total Fair Value	$ 531,917	$ 297,365
Assets, fair value	561,836	324,289
Fair Value, Inputs, Level 1 [Member]
Assets, fair value	43,388	61,259
Fair Value, Inputs, Level 2 [Member]
Assets, fair value	515,947	260,169
Fair Value, Inputs, Level 3 [Member]
Assets, fair value	2,501	2,861
Money Market Funds [Member]
Total Fair Value	13,469	34,335
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	13,469	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	139,361
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	139,361
Certificates of Deposit [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	0
Corporate Debt Securities [Member]
Total Fair Value	369,094	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	369,094	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	7,492	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	7,492	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	2,501	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	2,501	2,861
Mutual Fund [Member]
Total Fair Value	29,919	26,924
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	29,919	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