Note 11 - Fair Value Measurements - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Total Fair Value	$ 236,627	$ 248,953
Assets, fair value	257,500	267,820
Fair Value, Inputs, Level 1 [Member]
Assets, fair value	76,955	60,002
Fair Value, Inputs, Level 2 [Member]
Assets, fair value	177,255	204,577
Fair Value, Inputs, Level 3 [Member]
Assets, fair value	3,290	3,241
Money Market Funds [Member]
Total Fair Value	56,082	41,135
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	56,082	41,135
Corporate Debt Securities [Member]
Total Fair Value	143,453	170,909
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	143,453	170,909
US Government Agencies Short-term Debt Securities [Member]
Total Fair Value	32,202	32,068
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	32,202	32,068
Certificates of Deposit [Member]
Total Fair Value	1,600	1,600
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	1,600	1,600
Auction Rate Securities [Member]
Total Fair Value	3,290	3,241
Auction Rate Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	3,290	3,241
Mutual Fund [Member]
Total Fair Value	20,873	18,867
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	$ 20,873	$ 18,867