Investments & Fair Value Measurements (Schedule Of Financial Assets and Liabilities Carried At Fair Value) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 28, 2018	Sep. 28, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 790,787	$ 918,064
Cash and cash equivalents	790,787	918,063
Available-for-sale Debt Securities, Gross Unrealized Loss		(1)
Available-for-sale Debt Securities, Amortized Cost Basis	492,560	380,795
Available-for-sale Securities, Debt Securities	490,406	377,723
Investment Owned, Unrecognized Unrealized Depreciation	(2,392)	(3,170)
Cash Cash Equivalents And Investments Total	1,167,772	1,283,983
Financial assets carried at fair value	790,787	918,063
Restricted Cash and Cash Equivalents	8,999	7,187
Long-term investments	201,428	187,782
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	111,251	206,479
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(448)	(2,204)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	173,127	80,613
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,942)	(967)
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis	290,447	191,241
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	289,860	190,541
Available For Sale Securities Debt Maturities After One Through Two Years Amortized Cost	138,303	122,131
Available For Sale Securities Debt Maturities After One Through Two Years Fair Value	136,797	120,545
Available For Sale Securities Debt Maturities After Two Through Three Years Amortized Cost	63,810	67,423
Available For Sale Securities Debt Maturities After Two Through Three Years Fair Value	63,749	66,637
Investment Owned, at Cost	1,169,827	1,286,810
Investment Owned, Unrecognized Unrealized Appreciation	337	343
Available-for-sale Securities, Gross Unrealized Gain	0
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	788,791	911,456
Financial assets carried at fair value	813,768	935,306
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	1,996	6,607
Financial assets carried at fair value	353,221	348,322
Cash [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	676,484	905,660
Cash [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	676,484	905,660
Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	112,307	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	112,307	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
Available-for-sale Debt Securities, Gross 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
Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Gross Unrealized Gain	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]
Available-for-sale Debt Securities, Gross Unrealized Loss
Available-for-sale Debt Securities Gross Unrealized Gain		14
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,299	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	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	20,069	21,486
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(221)	(302)
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		9,625
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	(147)	(135)
Available-for-sale Securities, Debt Securities	15,351	11,862
Available-for-sale Debt Securities Gross Unrealized Gain
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	15,351	11,862
Government Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		2,495
Available-for-sale Debt Securities, Gross Unrealized Loss		(15)
Available-for-sale Debt Securities, Amortized Cost Basis		7,970
Available-for-sale Debt Securities Gross 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	5,455	16,633
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(37)	(332)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	9,519	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(157)	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
Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	1,996	5,058
Available-for-sale Debt Securities, Gross Unrealized Loss		0
Available-for-sale Debt Securities, Amortized Cost Basis		4,276
Available-for-sale Debt Securities Gross 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	7,047	5,737
Investment Owned, Unrecognized Unrealized Appreciation	1	1
Available-for-sale Securities, Gross Unrealized Loss	0
Commercial Paper [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	1,996	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
Available-for-sale Debt Securities, Gross Unrealized Loss		(494)
Available-for-sale Debt Securities, Amortized Cost Basis		111,245
Available-for-sale Debt Securities Gross 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	80,200	143,051
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(381)	(1,680)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	124,808	52,162
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,497)	(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		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]
Available-for-sale Debt Securities, Gross Unrealized Loss		(120)
Available-for-sale Debt Securities, Amortized Cost Basis		30,475
Available-for-sale Debt Securities Gross Unrealized Gain
Available-for-sale Securities, Amortized Cost Basis		17,235
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(29)	(191)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	18,731	6,965
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(67)	(41)
Investment Owned, at Cost	17,250	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]
Available-for-sale Debt Securities Gross 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	3,288	3,486
Financial liabilities carried at fair value	3,288	3,486
Short-term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Gross Unrealized Loss		(764)
Available-for-sale Debt Securities, Amortized Cost Basis		178,838
Available-for-sale Securities, Debt Securities		178,138
Short-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	10,385	7,955
Short-term Investments [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	165,172	170,183
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	(1)
Available-for-sale Debt Securities, Amortized Cost Basis	12,645	12,875
Available-for-sale Securities, Debt Securities	12,652
Available-for-sale Debt Securities Gross Unrealized Gain	8
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	12,652
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		11,997
Short-term Investments [Member] | US Government Agencies Short-term Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	15,498
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	(11)
Available-for-sale Debt Securities, Amortized Cost Basis	10,394
Available-for-sale Securities, Debt Securities	10,385	7,955
Available-for-sale Debt Securities Gross Unrealized Gain	2
Short-term Investments [Member] | Government Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	10,385	7,955
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	(1)
Available-for-sale Debt Securities, Amortized Cost Basis	8,450
Available-for-sale Securities, Debt Securities	8,450	4,276
Available-for-sale Debt Securities Gross Unrealized Gain	1
Short-term Investments [Member] | Commercial Paper [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	8,450
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
Available-for-sale Debt Securities, Gross Unrealized Loss	(384)
Available-for-sale Debt Securities, Amortized Cost Basis	100,814
Available-for-sale Securities, Debt Securities	100,435	110,801
Available-for-sale Debt Securities Gross Unrealized Gain	5
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	100,435
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	(60)
Available-for-sale Debt Securities, Amortized Cost Basis	28,343
Available-for-sale Securities, Debt Securities	28,284	30,355
Available-for-sale Debt Securities Gross Unrealized Gain	1
Short-term Investments [Member] | Municipal Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Fair Value Disclosure	28,284
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	(604)
Available-for-sale Debt Securities, Amortized Cost Basis	176,144
Available-for-sale Securities, Debt Securities	175,557
Available-for-sale Debt Securities Gross Unrealized Gain	17
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	14,592	15,895
Long-term Investments [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, Noncurrent, Fair Value Disclosure	186,053	171,532
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	0
Available-for-sale Securities, Amortized Cost Basis	430
Available-for-sale Securities	430
Available-for-sale Securities, Gross Unrealized Loss
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	6,791
Available-for-sale Securities	6,740	9,625
Available-for-sale Securities, Gross Unrealized Gain	23
Available-for-sale Securities, Gross Unrealized Loss	74	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	6,740
Long-term Investments [Member] | Government Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	14,673
Available-for-sale Securities	14,493	15,649
Available-for-sale Securities, Gross Unrealized Gain	4
Available-for-sale Securities, Gross Unrealized Loss	184	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	14,493
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	158,367
Available-for-sale Securities	157,026	144,784
Available-for-sale Securities, Gross Unrealized Gain	152	33
Available-for-sale Securities, Gross Unrealized Loss	1,493	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	157,026
Long-term Investments [Member] | Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities, Amortized Cost Basis	21,852
Available-for-sale Securities	21,857	17,123
Available-for-sale Securities, Gross Unrealized Gain	42
Available-for-sale Securities, Gross Unrealized Loss	37	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	21,857
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	783
Available-for-sale Securities	882	601
Available-for-sale Securities, Gross Unrealized Gain	99	246
Available-for-sale Securities, Gross Unrealized Loss
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	99
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	202,896
Available-for-sale Securities	201,428
Available-for-sale Securities, Gross Unrealized Gain	320
Available-for-sale Securities, Gross Unrealized Loss	$ 1,788