Investments and Fair Value Measurements (Unrealized Loss Position) (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 27, 2019	Sep. 28, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash		$ 918,064
Long-term investments, unrealized gains	$ 0
Short-term investments, cost	413,927	380,795
Debt Securities, Available-for-sale, Unrealized Loss	0	1
Short term investments, total	416,210	377,723
Fair Value	18,925	206,479
Gross Unrealized Losses	(16)	(2,204)
Greater than 12 months, Fair Value	32,765	80,613
Greater than 12 months, Gross Unrealized Losses	(41)	(967)
Investment Owned, at Cost	1,093,659	1,286,810
Investment Owned, Unrecognized Unrealized Appreciation	(2,341)	(343)
Long-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, unrealized gains		279
Available-for-sale Securities, Gross Unrealized Loss		2,405
Certificates of Deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	300	0
Gross Unrealized Losses	0	0
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
U.S. agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Gross Unrealized Losses	0	0
Greater than 12 months, Fair Value	4,787	21,486
Greater than 12 months, Gross Unrealized Losses	(9)	(302)
Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash		1,005
Fair Value	7,647	143,051
Gross Unrealized Losses	(3)	(1,680)
Greater than 12 months, Fair Value	27,078	52,162
Greater than 12 months, Gross Unrealized Losses	(32)	(624)
Municipal debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash		544
Gross Unrealized Losses	(13)	(191)
Greater than 12 months, Fair Value	900	6,965
Greater than 12 months, Gross Unrealized Losses	0	(41)
Investment Owned, at Cost	9,552	41,058
Government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash		2,495
Long-term investments, unrealized gains	0
Short-term investments, cost	653
Available-for-sale Securities, Gross Unrealized Loss	0
Short term investments, total	653
Fair Value	1,426	16,633
Gross Unrealized Losses	0	(332)
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash		5,058
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Investment Owned, at Cost	0	5,737
Investment Owned, Unrecognized Unrealized Appreciation	0	(1)
Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash	1,000
Debt Securities, Available-for-sale, Unrealized Loss	0
Short-term investments \| Certificates of Deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost	1,265	12,875
Debt Securities, Available-for-sale, Unrealized Loss	0	0
Short term investments, total	1,266	12,889
Short-term investments \| U.S. agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost		11,997
Short-term investments \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost	76,802	111,245
Debt Securities, Available-for-sale, Unrealized Loss	34	494
Short term investments, total	76,940	110,801
Short-term investments \| Municipal debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost	15,210	30,475
Debt Securities, Available-for-sale, Unrealized Loss	7	120
Short term investments, total	15,221	30,355
Short-term investments \| Government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost	8,381	7,970
Debt Securities, Available-for-sale, Unrealized Loss	1	15
Short term investments, total	8,391	7,955
Short-term investments \| Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments, cost	6,347	4,276
Debt Securities, Available-for-sale, Unrealized Loss	0	0
Short term investments, total	6,356	4,276
Long-term investments \| Long-term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, unrealized gains	2,122
Available-for-sale Securities, Gross Unrealized Loss	6
Long-term investments \| U.S. agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, unrealized gains	146	0
Available-for-sale Securities, Gross Unrealized Loss	0	166
Long-term investments \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, unrealized gains	1,700	33
Available-for-sale Securities, Gross Unrealized Loss	0	1,810
Long-term investments \| Municipal debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, unrealized gains	87	0
Available-for-sale Securities, Gross Unrealized Loss	6	112
Long-term investments \| Government bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term investments, unrealized gains	187	0
Available-for-sale Securities, Gross Unrealized Loss	$ 0	$ 317

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Cost of the investment.

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