Note 4 - Fair Value Measurement - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Total	$ 248,953	$ 229,221
Assets, fair value	267,820	245,846
Fair Value, Inputs, Level 1 [Member]
Assets, fair value	60,002	23,759
Fair Value, Inputs, Level 2 [Member]
Assets, fair value	204,577	216,831
Fair Value, Inputs, Level 3 [Member]
Assets, fair value	3,241	5,256
Money Market Funds [Member]
Available-for-sale Securities, Total	41,135	7,134
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale Securities, Total	41,135	7,134
Corporate Debt Securities [Member]
Available-for-sale Securities, Total	170,909	203,807
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale Securities, Total	170,909	203,807
US Government Agencies Short-term Debt Securities [Member]
Available-for-sale Securities, Total	32,068	13,024
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale Securities, Total	32,068	13,024
Certificates of Deposit [Member]
Available-for-sale Securities, Total	1,600
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale Securities, Total	1,600
Auction Rate Securities [Member]
Available-for-sale Securities, Total	3,241	5,256
Auction Rate Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale Securities, Total	3,241	5,256
Mutual Fund [Member]
Available-for-sale Securities, Total	18,867	16,625
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale Securities, Total	$ 18,867	$ 16,625