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Marketable Securities Schedule of Available-for-sale Securities (Details) - USD ($) $ in Millions | 9 Months Ended |
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Jun. 26, 2020 | Sep. 27, 2019
Debt Securities, Available-for-sale [Line Items] |  |
Document Period End Date | Jun. 26, 2020 |
US Treasury and Government [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | $ 0.5 |
Available-for-sale Securities | 69.3 |
Corporate Debt Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 0.2 |
Available-for-sale Securities | 261.1 |
US States and Political Subdivisions Debt Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities | $ 40.7 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | $ (0.1)

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- Definition The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements containing historical data, it is the date up through which that historical data is presented. If there is no historical data in the report, use the filing date. The format of the date is CCYY-MM-DD. + References No definition available. + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 + Details
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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 + Details
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