Investments and Fair Value Measurements (Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Sep. 29, 2017	Sep. 30, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 211,423	$ 223,583
Gross Unrealized Losses	(437)	(370)
Greater than 12 months, Fair Value	129,546	5,615
Greater than 12 months, Gross Unrealized Losses	(616)	(5)
Investment Owned, at Cost	1,189,392	1,030,769
Investment Owned, Unrecognized Unrealized Appreciation	(799)	(1,251)
Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	19,750	0
Gross Unrealized Losses	(6)	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	19,713	22,988
Gross Unrealized Losses	(91)	(38)
Greater than 12 months, Fair Value	11,386	0
Greater than 12 months, Gross Unrealized Losses	(108)	0
Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	125,890	153,491
Gross Unrealized Losses	(251)	(280)
Greater than 12 months, Fair Value	109,806	1,000
Greater than 12 months, Gross Unrealized Losses	(449)	0
Municipal debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Unrealized Losses	(24)	(42)
Greater than 12 months, Fair Value	3,625	4,615
Greater than 12 months, Gross Unrealized Losses	(10)	(5)
Investment Owned, at Cost	26,749	35,625
US Treasury and Government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	15,029	0
Gross Unrealized Losses	(64)	0
Greater than 12 months, Fair Value	4,729	0
Greater than 12 months, Gross Unrealized Losses	(49)	0
Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Greater than 12 months, Fair Value	0	0
Greater than 12 months, Gross Unrealized Losses	0	0
Investment Owned, at Cost	4,292	11,479
Investment Owned, Unrecognized Unrealized Appreciation	$ (1)	$ (10)

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 320
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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.

+ References

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- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Cost of the investment.

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-Publisher FASB
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-SubTopic 210
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This element represents the excess of the cost (face amount, notional amount) of an investment (security, contract) over its fair value which deficiency has not been recognized in earnings of the entity.

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