Investments & Fair Value Measurements (Schedule Of Financial Assets and Liabilities Carried At Fair Value) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Mar. 30, 2018	Sep. 29, 2017	Mar. 31, 2017	Sep. 30, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		$ 627,017
Cash and cash equivalents	$ 705,519	627,017	$ 532,508	$ 516,112
Available-for-sale Debt Securities, Amortized Cost Basis	565,329	561,977
Available-for-sale Securities, Debt Securities	561,548	561,366
Investment Owned, Unrecognized Unrealized Depreciation	(3,909)	(1,053)
Cash Cash Equivalents And Investments Total	1,208,078	1,189,138
Financial assets carried at fair value	705,519	627,017
Restricted Cash and Cash Equivalents	8,082	7,351
Long-term investments	262,062	314,364
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	322,698	211,423
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,622)	(437)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	132,411	129,546
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,286)	(616)
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis	304,019	251,649
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	303,376	251,530
Available For Sale Securities Debt Maturities After One Through Two Years Amortized Cost	138,508	213,555
Available For Sale Securities Debt Maturities After One Through Two Years Fair Value	137,073	213,154
Available For Sale Securities Debt Maturities After Two Through Three Years Amortized Cost	122,802	96,773
Available For Sale Securities Debt Maturities After Two Through Three Years Fair Value	121,099	96,682
Investment Owned, at Cost	1,211,587	1,189,392
Investment Owned, Unrecognized Unrealized Appreciation	400	799
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	2,416	2,550
Financial assets carried at fair value	18,324	22,667
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	60,463	1,223
Financial assets carried at fair value	543,495	539,056
Cash [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	642,640	623,244
Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	16,395
Cash Equivalents, at Carrying Value	16,398
Available-for-sale Debt Securities, Gross Unrealized Loss	(3)
Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	2,416	2,550
Cash and cash equivalents	2,416
Money Market Funds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	2,416	2,550
Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Gross Unrealized Loss		(1)
Available-for-sale Debt Securities Gross Unrealized Gain		9
Available-for-sale Securities, Amortized Cost Basis		22,940
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	15,288	19,750
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(5)	(6)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Certificates of Deposit [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure		17,244
Investments, Noncurrent, Fair Value Disclosure		22,939
U.S. Agency Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis		21,779
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	4,704	19,713
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(85)	(91)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	22,539	11,386
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(261)	(108)
U.S. Agency Securities [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure
U.S. Agency Securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure		21,601
US Government Agencies Short-term Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Gross Unrealized Loss	(22)	(20)
Available-for-sale Securities, Debt Securities	7,283	9,498
Available-for-sale Debt Securities Gross Unrealized Gain		0
US Government Agencies Short-term Debt Securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	7,283	9,498
Government Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Gross Unrealized Loss		(6)
Available-for-sale Debt Securities Gross Unrealized Gain		0
Available-for-sale Securities, Amortized Cost Basis		17,839
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	13,730	15,029
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(229)	(64)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,906	4,729
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(26)	(49)
Government Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure		17,732
Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	44,068	1,223
Cash and cash equivalents	44,065
Available-for-sale Debt Securities, Gross Unrealized Loss		(1)
Available-for-sale Debt Securities, Amortized Cost Basis		15,160
Available-for-sale Debt Securities Gross Unrealized Gain		2
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Investment Owned, at Cost	19,822	4,292
Investment Owned, Unrecognized Unrealized Appreciation	16	1
Available-for-sale Securities, Gross Unrealized Gain	4
Available-for-sale Securities, Gross Unrealized Loss	(1)
Commercial Paper [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	44,068	1,223
Investments, Fair Value Disclosure		15,161
Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		2,550
Available-for-sale Debt Securities, Gross Unrealized Loss		(163)
Available-for-sale Debt Securities, Amortized Cost Basis		174,750
Available-for-sale Debt Securities Gross Unrealized Gain		54
Available-for-sale Securities, Amortized Cost Basis		218,857
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	212,227	125,890
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,070)	(251)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	106,413	109,806
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(984)	(449)
Corporate Debt Securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	16,395
Investments, Fair Value Disclosure		174,641
Investments, Noncurrent, Fair Value Disclosure		218,647
Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Gross Unrealized Loss		(9)
Available-for-sale Debt Securities, Amortized Cost Basis		29,178
Available-for-sale Debt Securities Gross Unrealized Gain		16
Available-for-sale Securities, Amortized Cost Basis		28,913
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(217)	(24)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,553	3,625
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(15)	(10)
Investment Owned, at Cost	56,927	26,749
Municipal Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure		29,185
Investments, Noncurrent, Fair Value Disclosure		28,917
Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities Gross Unrealized Gain		81
Other Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cost Method Investment, Fair Value Measurement Adjustment	300	400
Available-for-sale Securities, Amortized Cost Basis		4,171
Investments, Noncurrent, Fair Value Disclosure	100	600
Other Long-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure		357
Investments Held In Supplemental Retirement Plan [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets carried at fair value	3,288	3,026
Financial liabilities carried at fair value	3,288	3,026
Cost Method Investment 1 [Member] | Other Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments	3,000	3,000
Cost Method Investment 2 [Member] | Other Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments	500	500
Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Gross Unrealized Loss		(200)
Available-for-sale Debt Securities, Amortized Cost Basis		247,876
Available-for-sale Securities, Debt Securities		247,757
Short-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	534	2,028
Short-term Investments [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	239,963	245,729
Short-term Investments [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Gross Unrealized Loss	(5)
Available-for-sale Debt Securities, Amortized Cost Basis	23,848	17,236
Available-for-sale Securities, Debt Securities	23,850
Available-for-sale Debt Securities Gross Unrealized Gain	7
Available-for-sale Securities		17,244
Short-term Investments [Member] | Certificates of Deposit [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	23,850
Short-term Investments [Member] | U.S. Agency Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	7,305	9,518
Short-term Investments [Member] | Government Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Gross Unrealized Loss	(1)
Available-for-sale Debt Securities, Amortized Cost Basis	535	2,034
Available-for-sale Securities, Debt Securities	534	2,028
Short-term Investments [Member] | Government Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	534	2,028
Short-term Investments [Member] | Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Gross Unrealized Loss	(16)
Available-for-sale Debt Securities, Amortized Cost Basis	8,575
Available-for-sale Securities, Debt Securities	8,559	15,161
Available-for-sale Debt Securities Gross Unrealized Gain	0
Short-term Investments [Member] | Commercial Paper [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	8,559
Short-term Investments [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Gross Unrealized Loss	(578)
Available-for-sale Debt Securities, Amortized Cost Basis	166,141
Available-for-sale Securities, Debt Securities	165,625	174,641
Available-for-sale Debt Securities Gross Unrealized Gain	62
Short-term Investments [Member] | Corporate Debt Securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	165,625
Short-term Investments [Member] | Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Gross Unrealized Loss	(90)
Available-for-sale Debt Securities, Amortized Cost Basis	34,736
Available-for-sale Securities, Debt Securities	34,646	29,185
Available-for-sale Debt Securities Gross Unrealized Gain	0
Short-term Investments [Member] | Municipal Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	34,646
Short-term Investments [Member] | Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Gross Unrealized Loss	(712)
Available-for-sale Debt Securities, Amortized Cost Basis	241,140
Available-for-sale Securities, Debt Securities	240,497
Available-for-sale Debt Securities Gross Unrealized Gain	69
Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis		314,499
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		718
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(853)
Available-for-sale Securities		314,364
Long-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	15,374	18,089
Long-term Investments [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	243,069	292,104
Long-term Investments [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities Gross Unrealized Gain	5
Available-for-sale Securities, Amortized Cost Basis	4,845
Available-for-sale Securities	4,850	22,939
Available-for-sale Securities, Gross Unrealized Gain		5
Available-for-sale Securities, Gross Unrealized Loss	0	6
Long-term Investments [Member] | Certificates of Deposit [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	4,850
Long-term Investments [Member] | U.S. Agency Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	20,284
Available-for-sale Securities	19,960	21,601
Available-for-sale Securities, Gross Unrealized Gain		0
Available-for-sale Securities, Gross Unrealized Loss	324	178
Long-term Investments [Member] | U.S. Agency Securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	19,960
Long-term Investments [Member] | Government Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	15,356
Available-for-sale Securities	15,103	17,732
Available-for-sale Securities, Gross Unrealized Gain		0
Available-for-sale Securities, Gross Unrealized Loss	253	107
Long-term Investments [Member] | Government Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	15,103
Long-term Investments [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	196,848
Available-for-sale Securities	194,426	218,647
Available-for-sale Securities, Gross Unrealized Gain	51	327
Available-for-sale Securities, Gross Unrealized Loss	2,473	537
Long-term Investments [Member] | Corporate Debt Securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	194,426
Long-term Investments [Member] | Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	23,976
Available-for-sale Securities	23,833	28,917
Available-for-sale Securities, Gross Unrealized Gain		29
Available-for-sale Securities, Gross Unrealized Loss	143	25
Long-term Investments [Member] | Municipal Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	23,833
Long-term Investments [Member] | Other Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	3,619
Available-for-sale Securities	3,890	4,528
Available-for-sale Securities, Gross Unrealized Gain	271	357
Available-for-sale Securities, Gross Unrealized Loss		$ 0
Long-term Investments [Member] | Other Long-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	271
Long-term Investments [Member] | Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	264,928
Available-for-sale Securities	262,062
Available-for-sale Securities, Gross Unrealized Gain	327
Available-for-sale Securities, Gross Unrealized Loss	$ 3,193