Note 9 - Fair Value Measurements - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Total Fair Value	$ 213,292	$ 185,831
Total Fair Value	226,389	197,939
Fair Value, Inputs, Level 1 [Member]
Total Fair Value	24,347	37,064
Fair Value, Inputs, Level 2 [Member]
Total Fair Value	196,674	155,521
Fair Value, Inputs, Level 3 [Member]
Total Fair Value	5,368	5,354
Money Market Funds [Member]
Total Fair Value	11,250	24,956
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	11,250	24,956
Corporate Debt Securities [Member]
Total Fair Value	180,551	109,644
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	180,551	109,644
US Government Agencies Short-term Debt Securities [Member]
Total Fair Value	16,123	45,877
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	16,123	45,877
Auction Rate Securities [Member]
Total Fair Value	5,368	5,354
Auction Rate Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	5,368	5,354
Mutual Fund [Member]
Total Fair Value	13,097	12,108
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	$ 13,097	$ 12,108