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	Mar. 31, 2018	Dec. 30, 2017
	$ 139,730	$ 155,615
US Treasury Securities [Member]
Short-term investments	9,904	8,431
Corporate Debt Securities [Member]
Short-term investments	25,330	22,014
US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments	992	1,496
Certificates of Deposit [Member]
Short-term investments	2,798
Debt Security, Government, Non-US [Member]
Short-term investments	595	619
Cash [Member]
Cash and cash equivalents	84,773	100,850
Money Market Funds [Member]
Cash and cash equivalents	15,338	22,205
Fair Value, Inputs, Level 1 [Member]
	84,773	100,850
Fair Value, Inputs, Level 1 [Member] | US Treasury 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] | Debt Security, Government, Non-US [Member]
Short-term investments
Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Cash and cash equivalents	84,773	100,850
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash and cash equivalents
Fair Value, Inputs, Level 2 [Member]
	54,957	54,765
Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Short-term investments	9,904	8,431
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Short-term investments	25,330	22,014
Fair Value, Inputs, Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member]
Short-term investments	992	1,496
Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Short-term investments	2,798
Fair Value, Inputs, Level 2 [Member] | Debt Security, Government, Non-US [Member]
Short-term investments	595	619
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	15,338	22,205
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] | 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] | Debt Security, Government, Non-US [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