Investments & Fair Value Measurements (Schedule Of Financial Assets and Liabilities Carried At Fair Value) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Jun. 28, 2019	Sep. 28, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		$ 918,064
Cash Equivalents, at Carrying Value	$ 716,558
Cash and cash equivalents	716,560	918,063
Debt Securities, Available-for-sale, Unrealized Loss	(1)	(1)
Debt Securities, Available-for-sale, Amortized Cost	392,626	380,795
Debt Securities, Available-for-sale	394,540	377,723
Investment Owned, Unrecognized Unrealized Depreciation	(233)	(3,170)
Cash Cash Equivalents And Investments Total	1,048,649	1,283,983
Financial assets carried at fair value	716,560	918,063
Restricted Cash and Cash Equivalents	7,485	7,187
Long-term investments	211,352	187,782
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	41,573	206,479
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(34)	(2,204)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	73,786	80,613
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(198)	(967)
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	189,056	191,241
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	189,039	190,541
Available For Sale Securities Debt Maturities After One Through Two Years Amortized Cost	125,979	122,131
Available For Sale Securities Debt Maturities After One Through Two Years Fair Value	126,737	120,545
Available For Sale Securities Debt Maturities After Two Through Three Years Amortized Cost	77,591	67,423
Available For Sale Securities Debt Maturities After Two Through Three Years Fair Value	78,764	66,637
Investment Owned, at Cost	1,046,736	1,286,810
Investment Owned, Unrecognized Unrealized Appreciation	2,146	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	683,867	911,456
Financial assets carried at fair value	717,849	935,306
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	32,693	6,607
Financial assets carried at fair value	328,956	348,322
Cash [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	652,269	905,660
Cash Equivalents, at Carrying Value	652,269
Cash [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	652,269	905,660
Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	6,264
Cash Equivalents, at Carrying Value	6,265
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	15,084	3,301
Cash Equivalents, at Carrying Value	15,084
Money Market Funds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	15,084	3,301
Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		544
Cash Equivalents, at Carrying Value	1,000
Cash and cash equivalents		545
Debt Securities, Available-for-sale, Unrealized Loss	0	(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
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,300	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	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
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	796	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3)	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	7,074	21,486
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(22)	(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	(6)	(135)
Debt Securities, Available-for-sale	6,276	11,862
Debt Securities, Available-for-sale, Unrealized Gain	10	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	6,276	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	16,513	7,970
Debt Securities, Available-for-sale	16,514
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	16,649	16,633
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	(332)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	8,647	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(36)	0
Available-for-sale Securities, Gross Unrealized Gain	1
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	16,514	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	0	5,737
Investment Owned, Unrecognized Unrealized Appreciation	0	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	20,145	143,051
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(30)	(1,680)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	53,766	52,162
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(137)	(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	6,264	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]
Cash and Cash Equivalents, Fair Value Disclosure	1,000
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	0	(191)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	4,299	6,965
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(3)	(41)
Investment Owned, at Cost	1,683	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	1,000	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		200
Available-for-sale Securities, Amortized Cost Basis		355
Investments, Noncurrent, Fair Value Disclosure	1,800	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	3,382	3,486
Financial liabilities carried at fair value	3,382	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	12,376
Debt Securities, Available-for-sale	12,376
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	12,376
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	13,051
Debt Securities, Available-for-sale	13,053
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	13,053
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	11,963	7,955
Short-term Investments [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	108,774	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	1,486	12,875
Debt Securities, Available-for-sale	1,487
Debt Securities, Available-for-sale, Unrealized Gain	1
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	1,487
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	6,272
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	11,952
Debt Securities, Available-for-sale	11,963	7,955
Debt Securities, Available-for-sale, Unrealized Gain	11
Short-term Investments [Member] | Government Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	11,963	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	0
Debt Securities, Available-for-sale, Amortized Cost	6,472
Debt Securities, Available-for-sale	6,485	4,276
Debt Securities, Available-for-sale, Unrealized Gain	13
Short-term Investments [Member] | Commercial Paper [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	6,485
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	(112)
Debt Securities, Available-for-sale, Amortized Cost	82,230
Debt Securities, Available-for-sale	82,169	110,801
Debt Securities, Available-for-sale, Unrealized Gain	51
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	82,169
Short-term Investments [Member] | Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	(2)
Debt Securities, Available-for-sale, Amortized Cost	12,345
Debt Securities, Available-for-sale	12,357	30,355
Debt Securities, Available-for-sale, Unrealized Gain	14
Short-term Investments [Member] | Municipal Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	12,357
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	(120)
Debt Securities, Available-for-sale, Amortized Cost	120,757
Debt Securities, Available-for-sale	120,737
Debt Securities, Available-for-sale, Unrealized Gain	100
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	22,019	15,895
Long-term Investments [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	187,489	171,532
Long-term Investments [Member] | Asset-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	7,799
Available-for-sale Securities	7,865
Available-for-sale Securities, Gross Unrealized Gain	66
Available-for-sale Securities, Gross Unrealized Loss	0
Long-term Investments [Member] | Asset-backed Securities [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	7,865
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	11,895
Available-for-sale Securities	12,010	9,625
Available-for-sale Securities, Gross Unrealized Gain	134	0
Available-for-sale Securities, Gross Unrealized Loss	(19)	(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	12,010
Long-term Investments [Member] | Government Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	21,871
Available-for-sale Securities	22,012	15,649
Available-for-sale Securities, Gross Unrealized Gain	177	0
Available-for-sale Securities, Gross Unrealized Loss	(36)	(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	22,012
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	154,196
Available-for-sale Securities	155,704	144,784
Available-for-sale Securities, Gross Unrealized Gain	1,563	33
Available-for-sale Securities, Gross Unrealized Loss	(55)	(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	155,704
Long-term Investments [Member] | Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	11,809
Available-for-sale Securities	11,910	17,123
Available-for-sale Securities, Gross Unrealized Gain	103	0
Available-for-sale Securities, Gross Unrealized Loss	(2)	(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	11,910
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	1,851
Available-for-sale Securities	1,851	601
Available-for-sale Securities, Gross Unrealized Gain	0	246
Available-for-sale Securities, Gross Unrealized Loss	0	$ 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	7
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	209,421
Available-for-sale Securities	211,352
Available-for-sale Securities, Gross Unrealized Gain	2,043
Available-for-sale Securities, Gross Unrealized Loss	$ (112)