Note 4 - Fair Value Measurements - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities	$ 229,221	$ 185,831
Total Fair Value	245,846	197,939
Fair Value, Inputs, Level 1 [Member]
Total Fair Value	23,759	37,064
Fair Value, Inputs, Level 2 [Member]
Total Fair Value	216,831	155,521
Fair Value, Inputs, Level 3 [Member]
Total Fair Value	5,256	5,354
Money Market Funds [Member]
Available-for-sale Securities	7,134	24,956
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale Securities	7,134	24,956
Corporate Debt Securities [Member]
Available-for-sale Securities	203,807	109,644
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale Securities	203,807	109,644
US Government Agencies Short-term Debt Securities [Member]
Available-for-sale Securities	13,024	45,877
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale Securities	13,024	45,877
Auction Rate Securities [Member]
Available-for-sale Securities	5,256	5,354
Auction Rate Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale Securities	5,256	5,354
Mutual Fund [Member]
Total Fair Value	16,625	12,108
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	$ 16,625	$ 12,108