Note 4 - Fair Value Measurements - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Total Fair Value	$ 297,365	$ 313,675
Assets, fair value	324,289	335,650
Fair Value, Inputs, Level 1 [Member]
Assets, fair value	61,259	50,075
Fair Value, Inputs, Level 2 [Member]
Assets, fair value	260,169	282,437
Fair Value, Inputs, Level 3 [Member]
Assets, fair value	2,861	3,138
Money Market Funds [Member]
Total Fair Value	34,335	28,100
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	34,335	28,100
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
Corporate Debt Securities [Member]
Total Fair Value	249,671	260,950
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	249,671	260,950
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	0	0
Commercial Paper [Member]
Total Fair Value	2,999	1,994
Commercial Paper [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	0	0
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	2,999	1,994
Commercial Paper [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	0	0
US Government Agencies Short-term Debt Securities [Member]
Total Fair Value	7,499	19,493
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	7,499	19,493
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	2,861	3,138
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	2,861	3,138
Mutual Fund [Member]
Total Fair Value	26,924	21,975
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	26,924	21,975
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