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Goodwill and Identifiable Assets Goodwill (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Apr. 02, 2018
Goodwill [Line Items] |  |  |  |
Amortization of intangible assets | $ 51.7 | $ 51.1 | $ 58.5 |
Goodwill [Roll Forward] |  |  |  |
Gross goodwill, period start | 1,486.9 | 1,421.1 |  |
Translation adjustment | 2.0 | (0.3) |  |
Gross goodwill, period end | 1,618.4 | 1,486.9 | 1,421.1 |
Apex |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Gross goodwill, period start | (703.5) | 662.1 |  |
Translation adjustment | (0.1) |  |  |
Gross goodwill, period end | (743.7) | (703.5) | 662.1 |
Oxford |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Gross goodwill, period start | (230.5) | 230.8 |  |
Translation adjustment | 2.1 | (0.3) |  |
Gross goodwill, period end | (232.6) | (230.5) | 230.8 |
ECS |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Gross goodwill, period start | 552.9 | 528.2 |  |
Gross goodwill, period end | 642.1 | 552.9 | $ 528.2 |
Business Acquisition, Goodwill, Expected Tax Deductible Amount |  |  |  | $ 514.2
Business Acquisition, Goodwill, Expected Tax Deductible Amount |  |  |  | $ 514.2
2019 Acquisitions |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Goodwill, Acquired During Period |  | 66.1 |  |
Business Acquisition, Goodwill, Expected Tax Deductible Amount |  | 63.3 |  |
Business Acquisition, Goodwill, Expected Tax Deductible Amount |  | 63.3 |  |
2019 Acquisitions | Apex |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Goodwill, Acquired During Period |  | 41.4 |  |
2019 Acquisitions | ECS |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Goodwill, Acquired During Period |  | $ 24.7 |  |
Other Acquisitions |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Goodwill, Acquired During Period | 129.5 |  |  |
Business Acquisition, Goodwill, Expected Tax Deductible Amount | 77.1 |  |  |
Business Acquisition, Goodwill, Expected Tax Deductible Amount | 77.1 |  |  |
Other Acquisitions | Apex |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Goodwill, Acquired During Period | 40.3 |  |  |
Other Acquisitions | ECS |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Goodwill, Acquired During Period | $ 89.2 |  |  |

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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274 + Details
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