Note 10 - Fair Value Measurements - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Total Fair Value	$ 251,756	$ 229,221
Assets, fair value	268,869	245,846
Fair Value, Inputs, Level 1 [Member]
Assets, fair value	33,194	23,759
Fair Value, Inputs, Level 2 [Member]
Assets, fair value	230,428	216,831
Fair Value, Inputs, Level 3 [Member]
Assets, fair value	5,247	5,256
Money Market Funds [Member]
Total Fair Value	16,081	7,134
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	16,081	7,134
Corporate Debt Securities [Member]
Total Fair Value	207,461	203,807
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	207,461	203,807
US Government Agencies Short-term Debt Securities [Member]
Total Fair Value	22,967	13,024
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	22,967	13,024
Auction Rate Securities [Member]
Total Fair Value	5,247	5,256
Auction Rate Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	5,247	5,256
Mutual Fund [Member]
Total Fair Value	17,113	16,625
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	$ 17,113	$ 16,625