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Goodwill and Identifiable Assets (Goodwill) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Goodwill [Roll Forward] |  |  |
Gross goodwill | $ 1,893.5 | $ 1,891.9 | $ 1,569.5
Goodwill, Purchase Accounting Adjustments | 1.1 | (8.1) |
Translation adjustment | 0.5 | (1.4) |
Commercial Business |  |  |
Goodwill [Roll Forward] |  |  |
Gross goodwill | 1,075.2 | 1,074.7 | 829.3
Goodwill, Purchase Accounting Adjustments | 0.0 | 0.4 |
Translation adjustment | 0.5 | (1.4) |
Federal Government Business |  |  |
Goodwill [Roll Forward] |  |  |
Gross goodwill | 818.3 | 817.2 | $ 740.2
Goodwill, Purchase Accounting Adjustments | 1.1 | (8.5) |
Translation adjustment | $ 0.0 | 0.0 |
2022 Acquisitions |  |  |
Goodwill [Roll Forward] |  |  |
Goodwill, Acquired During Period |  | 331.9 |
Business Acquisition, Goodwill, Expected Tax Deductible Amount |  | 250.7 |
2022 Acquisitions | Commercial Business |  |  |
Goodwill [Roll Forward] |  |  |
Goodwill, Acquired During Period |  | 246.4 |
2022 Acquisitions | Federal Government Business |  |  |
Goodwill [Roll Forward] |  |  |
Goodwill, Acquired During Period |  | $ 85.5 |

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- Definition The amount of goodwill arising from a business combination that is expected to be deductible for tax purposes. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479581/805-30-50-1 + Details
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- Definition Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 350 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 24 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482548/350-20-55-24 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 350 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482573/350-20-50-1 + Details
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- Definition Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 350 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482573/350-20-50-1 + Details
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