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Goodwill and Identifiable Assets (Goodwill) (Details) - USD ($) $ in Millions |  |  |  | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Aug. 31, 2020 | Jan. 24, 2020 | Oct. 17, 2019 | Sep. 30, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Goodwill [Roll Forward] |  |  |  |  |  |
Gross goodwill |  |  |  | $ 1,590.9 | $ 1,486.9 | $ 1,421.1
Translation adjustment |  |  |  | 0.5 | (0.3) |
Apex |  |  |  |  |  |
Goodwill [Roll Forward] |  |  |  |  |  |
Gross goodwill |  |  |  | 745.5 | 703.5 | 662.1
Translation adjustment |  |  |  | (0.4) |  |
Oxford |  |  |  |  |  |
Goodwill [Roll Forward] |  |  |  |  |  |
Gross goodwill |  |  |  | 231.4 | 230.5 | 230.8
Translation adjustment |  |  |  | 0.9 | (0.3) |
ECS |  |  |  |  |  |
Goodwill [Roll Forward] |  |  |  |  |  |
Gross goodwill |  |  |  | 614.0 | 552.9 | $ 528.2
DHA |  |  |  |  |  |
Goodwill [Roll Forward] |  |  |  |  |  |
Goodwill, Acquired During Period |  |  |  |  | 24.7 |
Intersys |  |  |  |  |  |
Goodwill [Roll Forward] |  |  |  |  |  |
Goodwill, Acquired During Period |  |  | $ 41.4 |  | $ 41.4 |
BlackStone Federal |  |  |  |  |  |
Goodwill [Roll Forward] |  |  |  |  |  |
Goodwill, Acquired During Period |  | $ 61.1 |  | 61.1 |  |
LeapFrog |  |  |  |  |  |
Goodwill [Roll Forward] |  |  |  |  |  |
Goodwill, Acquired During Period | $ 42.4 |  |  | $ 42.4 |  |

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267 + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267 + Details
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- Definition Amount before accumulated impairment loss of an asset representing future economic benefits arising 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 -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267 + Details
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