Note 3 - Financial Instruments Measured at Fair Value - Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
	$ 138,324	$ 128,035
US Treasury Securities [Member]
Short-term investments	4,596
Foreign Government Debt Securities [Member]
Short-term investments	603	623
Corporate Debt Securities [Member]
Short-term investments	19,329	24,108
US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments		8,108
Certificates of Deposit [Member]
Short-term investments	1,298	751
Cash [Member]
Cash and cash equivalents	98,678	70,279
Money Market Funds [Member]
Cash and cash equivalents	13,820	24,166
Fair Value, Inputs, Level 1 [Member]
	98,678	70,279
Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]
Short-term investments
Fair Value, Inputs, Level 1 [Member] | Foreign Government Debt Securities [Member]
Short-term investments
Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Short-term investments
Fair Value, Inputs, Level 1 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments
Fair Value, Inputs, Level 1 [Member] | Certificates of Deposit [Member]
Short-term investments
Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Cash and cash equivalents	98,678	70,279
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 2 [Member]
	39,646	57,756
Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Short-term investments	4,596
Fair Value, Inputs, Level 2 [Member] | Foreign Government Debt Securities [Member]
Short-term investments	603	623
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Short-term investments	19,329	24,108
Fair Value, Inputs, Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments		8,108
Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Short-term investments	1,298	751
Fair Value, Inputs, Level 2 [Member] | Cash [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Cash and cash equivalents	13,820	24,166
Fair Value, Inputs, Level 3 [Member]
Fair Value, Inputs, Level 3 [Member] | US Treasury Securities [Member]
Short-term investments
Fair Value, Inputs, Level 3 [Member] | Foreign Government Debt Securities [Member]
Short-term investments
Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Short-term investments
Fair Value, Inputs, Level 3 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments
Fair Value, Inputs, Level 3 [Member] | Certificates of Deposit [Member]
Short-term investments
Fair Value, Inputs, Level 3 [Member] | Cash [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Cash and cash equivalents