Note 4 - Financial Instruments Measured at Fair Value - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 24, 2016	Dec. 26, 2015
Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Assets fair value, recurring	$ 58,299	$ 73,746
Fair Value, Inputs, Level 1 [Member]
Assets fair value, recurring	58,299	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	19,952
Fair Value, Inputs, Level 2 [Member] | Foreign Government Debt Securities [Member]
Assets fair value, recurring	639	650
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Assets fair value, recurring	42,450	41,624
Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Assets fair value, recurring	3,314	1,002
Fair Value, Inputs, Level 2 [Member]
Assets fair value, recurring	70,369	43,276
Cash [Member]
Assets fair value, recurring	58,299	73,746
Corporate Debt Securities [Member]
Assets fair value, recurring	19,952
Foreign Government Debt Securities [Member]
Assets fair value, recurring	639	650
US Government-sponsored Enterprises Debt Securities [Member]
Assets fair value, recurring	4,014
Money Market Funds [Member]
Assets fair value, recurring	42,450	41,624
Certificates of Deposit [Member]
Assets fair value, recurring	3,314	1,002
Assets fair value, recurring	$ 128,668	$ 117,022