Note 4 - Fair Value Measurements - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities	$ 185,831	$ 182,107
Total Fair Value	197,939	190,386
Fair Value, Inputs, Level 1 [Member]
Total Fair Value	37,064	39,919
Fair Value, Inputs, Level 2 [Member]
Total Fair Value	155,521	145,106
Fair Value, Inputs, Level 3 [Member]
Total Fair Value	5,354	5,361
Money Market Funds [Member]
Available-for-sale Securities	24,956	31,640
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale Securities	24,956	31,640
Corporate Debt Securities [Member]
Available-for-sale Securities	109,644
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale Securities	109,644
US Government Agencies Short-term Debt Securities [Member]
Available-for-sale Securities	45,877	123,532
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale Securities	45,877	123,532
Auction Rate Securities [Member]
Available-for-sale Securities	5,354	5,361
Auction Rate Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale Securities	5,354	5,361
Mutual Fund [Member]
Total Fair Value	12,108	8,279
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	$ 12,108	8,279
Certificates of Deposit [Member]
Available-for-sale Securities		21,574
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale Securities		$ 21,574