Investment Securities (Available For Sale Investments) (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 26, 2015	Dec. 27, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 179,628	$ 161,817
Gross Unrealized Gains	[1]	1,286	650
Gross Unrealized Losses		(127)	(344)
Fair Value		180,787	162,123
Accrued interest on investments		600	600
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		4,040	8,729
Fair Value		4,040	8,729
Cash Equivalents [Member] | Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		4,040	8,729
Fair Value		4,040	8,729
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		41,252	45,288
Gross Unrealized Gains	[1]	1,102	613
Gross Unrealized Losses		(7)	(16)
Fair Value		42,347	45,885
Short-Term Investments [Member] | Corporate Bonds And Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		32,879	43,777
Gross Unrealized Gains	[1]	958	498
Gross Unrealized Losses		(5)	(16)
Fair Value		33,832	44,259
Short-Term Investments [Member] | U.S. Treasury And Government Agency Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		5,131	1,010
Gross Unrealized Gains	[1]	92	79
Fair Value		5,223	1,089
Short-Term Investments [Member] | Foreign Government And Agency Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		3,012	501
Gross Unrealized Gains	[1]	51	36
Gross Unrealized Losses		(2)
Fair Value		3,061	537
Short-Term Investments [Member] | US States And Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		230
Gross Unrealized Gains	[1]	1
Fair Value		231
Long-Term Investment Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		134,336	107,800
Gross Unrealized Gains	[1]	184	37
Gross Unrealized Losses		(120)	(328)
Fair Value		134,400	107,509
Long-Term Investment Securities [Member] | Corporate Bonds And Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		84,866	61,357
Gross Unrealized Gains	[1]	75	28
Gross Unrealized Losses		(115)	(213)
Fair Value		84,826	61,172
Long-Term Investment Securities [Member] | U.S. Treasury And Government Agency Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		41,508	38,650
Gross Unrealized Gains	[1]	105	7
Gross Unrealized Losses		(1)	(76)
Fair Value		41,612	38,581
Long-Term Investment Securities [Member] | Foreign Government And Agency Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		7,962	7,563
Gross Unrealized Gains	[1]	4	1
Gross Unrealized Losses		(4)	(39)
Fair Value		$ 7,962	7,525
Long-Term Investment Securities [Member] | US States And Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			230
Gross Unrealized Gains	[1]		1
Fair Value			$ 231

[1] Gross unrealized gains include accrued interest on investments of $0.6 million which are included in the Consolidated Statement of Operations. The remainder of the gross unrealized gains and losses are included in the Consolidated Balance Sheet as Accumulated other comprehensive income (loss).