Note 4 - Fair Value Measurements - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Total Fair Value	$ 313,675	$ 248,953
Assets, fair value	335,650	267,820
Fair Value, Inputs, Level 1 [Member]
Assets, fair value	50,075	60,002
Fair Value, Inputs, Level 2 [Member]
Assets, fair value	282,437	204,577
Fair Value, Inputs, Level 3 [Member]
Assets, fair value	3,138	3,241
Money Market Funds [Member]
Total Fair Value	28,100	41,135
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	28,100	41,135
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	260,950	170,909
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	260,950	170,909
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	0	0
Commercial Paper [Member]
Total Fair Value	1,994
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	1,994
Commercial Paper [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	0
US Government Agencies Short-term Debt Securities [Member]
Total Fair Value	19,493	32,068
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	19,493	32,068
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	0	0
Certificates of Deposit [Member]
Total Fair Value		1,600
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		1,600
Certificates of Deposit [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value		0
Auction Rate Securities [Member]
Total Fair Value	3,138	3,241
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	3,138	3,241
Mutual Fund [Member]
Total Fair Value	21,975	18,867
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	21,975	18,867
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