Note 3 - Cash, Cash Equivalents and Investments - Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Adjusted Cost	$ 230,585	$ 186,854
Available-for-sale Securities	229,221	185,831
Unrealized Gains	17	65
Unrealized Losses	(1,381)	(1,088)
Fair Value of Investments in Unrealized Loss Position	215,844	136,567
Money Market Funds [Member]
Adjusted Cost	7,134	24,956
Available-for-sale Securities	7,134	24,956
Corporate Debt Securities [Member]
Adjusted Cost	204,789	110,429
Available-for-sale Securities	203,807	109,644
Unrealized Gains	17	65
Unrealized Losses	(999)	(850)
Fair Value of Investments in Unrealized Loss Position	197,564	91,938
US Government Agencies Short-term Debt Securities [Member]
Adjusted Cost	13,092	45,899
Available-for-sale Securities	13,024	45,877
Unrealized Gains
Unrealized Losses	(68)	(22)
Fair Value of Investments in Unrealized Loss Position	13,024	39,275
Auction Rate Securities [Member]
Adjusted Cost	5,570	5,570
Available-for-sale Securities	5,256	5,354
Unrealized Losses	(314)	(216)
Fair Value of Investments in Unrealized Loss Position	$ 5,256	$ 5,354

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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