Note 2 - Investment Securities (Details) - Financial Assets Measured at Fair Value on a Recurring Basis (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Short term marketable investments:
Available-for-sale securities	$ 71,343	$ 66,831
Total assets at fair value	73,385	79,257
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash equivalents:
Cash and cash equivalents	242	9,926
Fair Value, Inputs, Level 1 [Member]
Short term marketable investments:
Total assets at fair value	242	9,926
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Cash equivalents:
Cash and cash equivalents	1,800	2,500
Fair Value, Inputs, Level 2 [Member]
Short term marketable investments:
Available-for-sale securities	71,343	66,831
Total assets at fair value	73,143	69,331
Money Market Funds [Member]
Cash equivalents:
Cash and cash equivalents	242	9,926
Commercial Paper [Member]
Cash equivalents:
Cash and cash equivalents	$ 1,800	$ 2,500