Note 4 - Financial Instruments Measured at Fair Value - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 26, 2015	Dec. 27, 2014
Cash [Member] | Fair Value, Inputs, Level 1 [Member]
Assets	$ 84,642	$ 66,467
Cash [Member] | Fair Value, Inputs, Level 2 [Member]
Assets
Cash [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Cash [Member]
Assets	$ 84,642	$ 66,467
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Assets
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Assets	$ 4,618	$ 4,418
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Money Market Funds [Member]
Assets	$ 4,618	$ 4,418
Certificates of Deposit [Member] | Fair Value, Inputs, Level 1 [Member]
Assets
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Assets	$ 1,002	$ 1,000
Certificates of Deposit [Member] | Fair Value, Inputs, Level 3 [Member]
Assets
Certificates of Deposit [Member]
Assets	$ 1,002	$ 1,000
Fair Value, Inputs, Level 1 [Member] | Municipal Bonds [Member]
Assets
Fair Value, Inputs, Level 1 [Member] | Foreign Corporate Debt Securities [Member]
Assets
Fair Value, Inputs, Level 1 [Member]
Assets	$ 84,642	$ 66,467
Fair Value, Inputs, Level 2 [Member] | Municipal Bonds [Member]
Assets		155
Fair Value, Inputs, Level 2 [Member] | Foreign Corporate Debt Securities [Member]
Assets	465
Fair Value, Inputs, Level 2 [Member]
Assets	$ 6,085	$ 5,573
Fair Value, Inputs, Level 3 [Member] | Municipal Bonds [Member]
Assets
Fair Value, Inputs, Level 3 [Member] | Foreign Corporate Debt Securities [Member]
Assets
Fair Value, Inputs, Level 3 [Member]
Assets
Municipal Bonds [Member]
Assets		$ 155
Foreign Corporate Debt Securities [Member]
Assets	$ 465
Assets	$ 90,727	$ 72,040