Investments (Maturities of Marketable Securities) (Details) - USD ($) $ in Thousands	Sep. 26, 2015	Dec. 27, 2014
Investments [Abstract]
Amortized Cost, Due within one year	$ 6,455
Fair Value, Due within one year	6,514
Amortized Cost, Due after one year through five years	53,775
Fair Value, Due after one year through five years	54,803
Amortized Cost, Due after five years through ten years	20,729
Fair Value, Due after five years through ten years	21,026
Amortized Cost, Equity securities	1,198
Fair Value, Equity securities	7,405
Amortized Cost	82,157	$ 65,759
Available-for-sale Securities, Fair Value, Total	$ 89,748	$ 73,959