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	Jun. 24, 2017	Dec. 31, 2016
	$ 114,282	$ 128,035
US Treasury Securities [Member]
Short-term investments	5,196
Foreign Government Debt Securities [Member]
Short-term investments	616	623
Corporate Debt Securities [Member]
Short-term investments	18,202	24,108
US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments	1,000	8,108
Certificates of Deposit [Member]
Short-term investments	1,294	751
Cash [Member]
Cash and cash equivalents	73,734	70,279
Money Market Funds [Member]
Cash and cash equivalents	14,240	24,166
Fair Value, Inputs, Level 1 [Member]
	73,734	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	73,734	70,279
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 2 [Member]
	40,548	57,756
Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Short-term investments	5,196
Fair Value, Inputs, Level 2 [Member] | Foreign Government Debt Securities [Member]
Short-term investments	616	623
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Short-term investments	18,202	24,108
Fair Value, Inputs, Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments	1,000	8,108
Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Short-term investments	1,294	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	14,240	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