Note 4 - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 25, 2016	Dec. 26, 2015
Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Assets fair value, recurring	$ 65,878	$ 73,746
Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Assets fair value, recurring
Fair Value, Inputs, Level 1 [Member] | Foreign Government Debt Securities [Member]
Assets fair value, recurring
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Assets fair value, recurring
Fair Value, Inputs, Level 1 [Member] | Certificates of Deposit [Member]
Assets fair value, recurring
Fair Value, Inputs, Level 1 [Member]
Assets fair value, recurring	65,878	73,746
Fair Value, Inputs, Level 2 [Member] | Cash [Member]
Assets fair value, recurring
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Assets fair value, recurring	15,045
Fair Value, Inputs, Level 2 [Member] | Foreign Government Debt Securities [Member]
Assets fair value, recurring	655	650
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Assets fair value, recurring	35,437	41,624
Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Assets fair value, recurring	913	1,002
Fair Value, Inputs, Level 2 [Member]
Assets fair value, recurring	52,050	43,276
Fair Value, Inputs, Level 3 [Member] | Cash [Member]
Assets fair value, recurring
Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Assets fair value, recurring
Fair Value, Inputs, Level 3 [Member] | Foreign Government Debt Securities [Member]
Assets fair value, recurring
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Assets fair value, recurring
Fair Value, Inputs, Level 3 [Member] | Certificates of Deposit [Member]
Assets fair value, recurring
Fair Value, Inputs, Level 3 [Member]
Assets fair value, recurring
Cash [Member]
Assets fair value, recurring	65,878	73,746
Corporate Debt Securities [Member]
Assets fair value, recurring	15,795
Foreign Government Debt Securities [Member]
Assets fair value, recurring	655	650
Money Market Funds [Member]
Assets fair value, recurring	35,437	41,624
Certificates of Deposit [Member]
Assets fair value, recurring	163	1,002
Assets fair value, recurring	$ 117,928	$ 117,022