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. 29, 2019	Sep. 28, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 726,594	$ 918,064
Cash and cash equivalents	726,596	918,063
Debt Securities, Available-for-sale, Unrealized Loss		(1)
Debt Securities, Available-for-sale, Amortized Cost	385,514	380,795
Debt Securities, Available-for-sale	385,393	377,723
Investment Owned, Unrecognized Unrealized Depreciation	(1,052)	(3,170)
Cash Cash Equivalents And Investments Total	1,084,577	1,283,983
Financial assets carried at fair value	726,596	918,063
Restricted Cash and Cash Equivalents	8,270	7,187
Long-term investments	199,302	187,782
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	49,453	206,479
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(110)	(2,204)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	135,979	80,613
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(943)	(967)
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	187,239	191,241
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	186,939	190,541
Available For Sale Securities Debt Maturities After One Through Two Years Amortized Cost	138,904	122,131
Available For Sale Securities Debt Maturities After One Through Two Years Fair Value	138,429	120,545
Available For Sale Securities Debt Maturities After Two Through Three Years Amortized Cost	59,371	67,423
Available For Sale Securities Debt Maturities After Two Through Three Years Fair Value	60,025	66,637
Investment Owned, at Cost	1,084,697	1,286,810
Investment Owned, Unrecognized Unrealized Appreciation	932	343
Available-for-sale Securities, Gross Unrealized Gain	3
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	702,345	911,456
Financial assets carried at fair value	739,961	935,306
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	24,251	6,607
Financial assets carried at fair value	344,616	348,322
Cash [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	698,336	905,660
Cash [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	698,336	905,660
Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	13,917
Debt Securities, Available-for-sale, Unrealized Loss	(1)
Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	3,845	3,301
Money Market Funds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	3,845	3,301
Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		544
Cash and cash equivalents		545
Debt Securities, Available-for-sale, Unrealized Loss		(1)
Municipal Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		0
Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		3,301
Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss		0
Debt Securities, Available-for-sale, Unrealized Gain		14
Certificates of Deposit [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure		12,889
U.S. Agency Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis		9,791
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	20,166	21,486
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(127)	(302)
U.S. Agency Securities [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure		0
U.S. Agency Securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure		9,625
US Government Agencies Short-term Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	(78)	(135)
Debt Securities, Available-for-sale	18,592	11,862
Debt Securities, Available-for-sale, Unrealized Gain	4	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	18,592	11,862
Government Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		2,495
Debt Securities, Available-for-sale, Unrealized Loss		(15)
Debt Securities, Available-for-sale, Amortized Cost	164	7,970
Debt Securities, Available-for-sale	164
Debt Securities, Available-for-sale, Unrealized Gain		0
Available-for-sale Securities, Amortized Cost Basis		15,966
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3,982	16,633
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(19)	(332)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	8,580	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(100)	0
Available-for-sale Securities, Gross Unrealized Gain	0
Available-for-sale Securities, Gross Unrealized Loss	0
Government Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		2,495
Investments, Noncurrent, Fair Value Disclosure		15,649
Government Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		5,058
Debt Securities, Available-for-sale, Unrealized Loss		0
Debt Securities, Available-for-sale, Amortized Cost		4,276
Debt Securities, Available-for-sale, Unrealized Gain		0
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	981	5,737
Investment Owned, Unrecognized Unrealized Appreciation	1	1
Commercial Paper [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		5,058
Investments, Fair Value Disclosure		4,276
Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		1,005
Debt Securities, Available-for-sale, Unrealized Loss		(494)
Debt Securities, Available-for-sale, Amortized Cost		111,245
Debt Securities, Available-for-sale, Unrealized Gain		50
Available-for-sale Securities, Amortized Cost Basis		146,561
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	39,957	143,051
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(89)	(1,680)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	101,333	52,162
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(700)	(624)
Corporate Debt Securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	13,917	1,005
Investments, Fair Value Disclosure		110,801
Investments, Noncurrent, Fair Value Disclosure		144,784
Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss		(120)
Debt Securities, Available-for-sale, Amortized Cost		30,475
Debt Securities, Available-for-sale, Unrealized Gain		0
Available-for-sale Securities, Amortized Cost Basis		17,235
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	(191)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5,900	6,965
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(16)	(41)
Investment Owned, at Cost	4,533	41,058
Municipal Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		544
Investments, Fair Value Disclosure		30,355
Investments, Noncurrent, Fair Value Disclosure		17,123
Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Gain		64
Other Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cost Method Investment, Fair Value Measurement Adjustment	100	200
Available-for-sale Securities, Amortized Cost Basis		355
Investments, Noncurrent, Fair Value Disclosure	800	400
Other Long-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure		246
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	3,355	3,486
Financial liabilities carried at fair value	3,355	3,486
Cash Equivalents [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	0
Debt Securities, Available-for-sale, Amortized Cost	1,100
Debt Securities, Available-for-sale	1,100
Debt Securities, Available-for-sale, Unrealized Gain	0
Cash Equivalents [Member] | Certificates of Deposit [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	1,100
Cash Equivalents [Member] | Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	0
Debt Securities, Available-for-sale, Amortized Cost	9,232
Debt Securities, Available-for-sale	9,234
Debt Securities, Available-for-sale, Unrealized Gain	2
Cash Equivalents [Member] | Commercial Paper [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	9,234
Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss		(764)
Debt Securities, Available-for-sale, Amortized Cost		178,838
Debt Securities, Available-for-sale		178,138
Short-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	12,084	7,955
Short-term Investments [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	146,595	170,183
Short-term Investments [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	0
Debt Securities, Available-for-sale, Amortized Cost	7,601	12,875
Debt Securities, Available-for-sale	7,604
Debt Securities, Available-for-sale, Unrealized Gain	3
Available-for-sale Securities		12,889
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	7,604
Short-term Investments [Member] | U.S. Agency Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Amortized Cost		11,997
Short-term Investments [Member] | US Government Agencies Short-term Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	18,666
Short-term Investments [Member] | Government Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	0
Debt Securities, Available-for-sale, Amortized Cost	12,078
Debt Securities, Available-for-sale	12,084	7,955
Debt Securities, Available-for-sale, Unrealized Gain	6
Short-term Investments [Member] | Government Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	12,084	7,955
Short-term Investments [Member] | Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	(1)
Debt Securities, Available-for-sale, Amortized Cost	8,389
Debt Securities, Available-for-sale	8,392	4,276
Debt Securities, Available-for-sale, Unrealized Gain	4
Short-term Investments [Member] | Commercial Paper [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	8,392
Short-term Investments [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		1,005
Debt Securities, Available-for-sale, Unrealized Loss	(263)
Debt Securities, Available-for-sale, Amortized Cost	99,968
Debt Securities, Available-for-sale	99,734	110,801
Debt Securities, Available-for-sale, Unrealized Gain	29
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	99,734
Short-term Investments [Member] | Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	(10)
Debt Securities, Available-for-sale, Amortized Cost	12,278
Debt Securities, Available-for-sale	12,273	30,355
Debt Securities, Available-for-sale, Unrealized Gain	5
Short-term Investments [Member] | Municipal Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	12,273
Short-term Investments [Member] | Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	(352)
Debt Securities, Available-for-sale, Amortized Cost	158,980
Debt Securities, Available-for-sale	158,679
Debt Securities, Available-for-sale, Unrealized Gain	51
Long-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis		189,908
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		279
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		(2,405)
Available-for-sale Securities		187,782
Long-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	25,532	15,895
Long-term Investments [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	173,770	171,532
Long-term Investments [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Gain	0
Available-for-sale Securities, Amortized Cost Basis	430
Available-for-sale Securities	430
Available-for-sale Securities, Gross Unrealized Loss	0
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	430
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	8,403
Available-for-sale Securities	8,401	9,625
Available-for-sale Securities, Gross Unrealized Gain	47	0
Available-for-sale Securities, Gross Unrealized Loss	49	166
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	8,401
Long-term Investments [Member] | Government Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	25,524
Available-for-sale Securities	25,468	15,649
Available-for-sale Securities, Gross Unrealized Gain	63	0
Available-for-sale Securities, Gross Unrealized Loss	119	317
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	25,468
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	153,558
Available-for-sale Securities	153,735	144,784
Available-for-sale Securities, Gross Unrealized Gain	701	33
Available-for-sale Securities, Gross Unrealized Loss	524	1,810
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	153,735
Long-term Investments [Member] | Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	10,360
Available-for-sale Securities	10,420	17,123
Available-for-sale Securities, Gross Unrealized Gain	67	0
Available-for-sale Securities, Gross Unrealized Loss	7	112
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	10,420
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	848
Available-for-sale Securities	848	601
Available-for-sale Securities, Gross Unrealized Gain	0	246
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	64
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	199,123
Available-for-sale Securities	199,302
Available-for-sale Securities, Gross Unrealized Gain	878
Available-for-sale Securities, Gross Unrealized Loss	$ 699