Note 10 - Fair Value Measurements - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Total Fair Value	$ 247,246	$ 229,221
Assets, fair value	263,530	245,846
Fair Value, Inputs, Level 1 [Member]
Assets, fair value	41,519	23,759
Fair Value, Inputs, Level 2 [Member]
Assets, fair value	216,754	216,831
Fair Value, Inputs, Level 3 [Member]
Assets, fair value	5,257	5,256
Money Market Funds [Member]
Total Fair Value	25,235	7,134
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	25,235	7,134
Corporate Debt Securities [Member]
Total Fair Value	190,924	203,807
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	190,924	203,807
US Government Agencies Short-term Debt Securities [Member]
Total Fair Value	24,230	13,024
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	24,230	13,024
Certificates of Deposit [Member]
Total Fair Value	1,600
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	1,600
Auction Rate Securities [Member]
Total Fair Value	5,257	5,256
Auction Rate Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	5,257	5,256
Mutual Fund [Member]
Total Fair Value	16,284	16,625
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	$ 16,284	$ 16,625