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Acquisitions Acquisition Costs, by Acquisition (Details) - USD ($) $ in Millions |  |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Oct. 17, 2019 | Jan. 25, 2019 | Apr. 02, 2018 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | [1] | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  | $ 1,486.9 |  |  |  | $ 1,421.1 |  |  |  | $ 1,486.9 | $ 1,421.1 |
Goodwill, Acquired During Period |  |  |  |  |  |  |  |  |  |  |  |  | 528.2 |
Revenues |  |  |  | $ 1,025.2 | $ 1,002.7 | $ 972.3 | $ 923.7 | $ 929.7 | $ 906.4 | $ 878.5 | $ 685.2 | 3,923.9 | 3,399.8 | $ 2,626.0
Intersys |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition, Effective Date of Acquisition | Oct. 17, 2019 |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition, Name of Acquired Entity | Intersys Consulting, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Businesses, Gross | $ 67.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition, Description of Acquired Entity | headquartered in Austin, Texas |  |  |  |  |  |  |  |  |  |  |  |  |
Business Combination, Reason for Business Combination | The acquisition expands the Company's capabilities in digital innovation and enterprise solutions |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill, Acquired During Period | $ 41.4 |  |  |  |  |  |  |  |  |  |  | 41.4 |  |
Business Acquisition, Goodwill, Expected Tax Deductible Amount | 38.7 |  |  |  |  |  |  |  |  |  |  |  |  |
Identifiable intangible assets | $ 23.8 |  |  |  |  |  |  |  |  |  |  |  |  |
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life | 9 years |  |  |  |  |  |  |  |  |  |  |  |  |
DHA |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition, Effective Date of Acquisition |  | Jan. 25, 2019 |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition, Name of Acquired Entity |  | DHA Group, Inc |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Businesses, Gross |  | $ 46.0 |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition, Description of Acquired Entity |  | headquartered in Washington, D.C. |  |  |  |  |  |  |  |  |  |  |  |
Business Combination, Reason for Business Combination |  | DHA was acquired to expand the Company's government services |  |  |  |  |  |  |  |  |  |  |  |
Goodwill, Acquired During Period |  | $ 24.7 |  |  |  |  |  |  |  |  |  | 24.7 |  |
Identifiable intangible assets |  | $ 19.0 |  |  |  |  |  |  |  |  |  |  |  |
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life |  | 9 years |  |  |  |  |  |  |  |  |  |  |  |
Apex |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  |  | 2,520.0 | 2,300.3 | 2,037.2
Apex | Intersys |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill, Acquired During Period |  |  |  |  |  |  |  |  |  |  |  | 41.4 |  |
ECS |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition, Effective Date of Acquisition |  |  | Apr. 02, 2018 |  |  |  |  |  |  |  |  |  |  |
Business Acquisition, Name of Acquired Entity |  |  | ECS Federal, LLC |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Businesses, Gross |  |  | $ 775.0 |  |  |  |  |  |  |  |  |  |  |
Business Acquisition, Transaction Costs |  |  | $ 12.0 |  |  |  |  |  |  |  |  |  |  |
Business Acquisition, Description of Acquired Entity |  |  | headquartered in Fairfax, Virginia |  |  |  |  |  |  |  |  |  |  |
Business Combination, Reason for Business Combination |  |  | The ECS acquisition allows the Company to compete in the federal IT and professional services sector |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  | $ 528.2 |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  |  | 798.2 | 493.0 | 0.0
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual |  |  |  |  |  |  |  |  |  |  |  | 798.2 | 493.0 | $ 0.0
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual |  |  |  |  |  |  |  |  |  |  |  |  | $ 14.2 |
Business Acquisition, Goodwill, Expected Tax Deductible Amount |  |  | 514.2 |  |  |  |  |  |  |  |  |  |  |
Identifiable intangible assets |  |  | $ 195.0 |  |  |  |  |  |  |  |  |  |  |
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life |  |  | 11 years |  |  |  |  |  |  |  |  |  |  |
ECS | DHA |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill, Acquired During Period |  |  |  |  |  |  |  |  |  |  |  | $ 24.7 |  |
[1] | ECS was acquired on April 2, 2018, see Note 4 – Acquisitions.

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