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Acquisitions Business Combinations Additional Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions |  |  | 9 Months Ended |
--- | --- | --- | --- | ---
Jan. 25, 2019 | Apr. 02, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
Business Acquisition [Line Items] |  |  |  |  |
Cash paid for acquisitions, net of cash acquired |  |  | $ 48.5 | $ 760.3 |
Goodwill |  |  | 1,444.7 |  | $ 1,421.1
Goodwill, Acquired During Period |  |  | $ 24.7 | 528.2 |
DHA |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business Acquisition, Effective Date of Acquisition | Jan. 25, 2019 |  |  |  |
Business Acquisition, Name of Acquired Entity | DHA Group, Inc. |  |  |  |
Business Acquisition, Description of Acquired Entity | headquartered in Washington, D.C. |  |  |  |
Working capital excess of target working capital | $ 2.5 |  |  |  |
Goodwill | 24.7 |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets | $ 19.0 |  |  |  |
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, 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 |  |  |
Business Acquisition, Goodwill, Expected Tax Deductible Amount |  | 514.2 |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets |  | $ 195.0 |  |  |
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life |  | 11 years |  |  |
Business Acquisition, Pro Forma Revenue |  |  |  | 2,619.2 |
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax |  |  |  | 123.7 |
Business Acquisition, Pro Forma Net Income (Loss) |  |  |  | $ 123.4 |
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax, Per Share, Basic |  |  |  | $ 2.36 |
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax, Per Share, Diluted |  |  |  | $ 2.32 |
Weighted Average Basic Shares Outstanding, Pro Forma |  |  |  | 52.3 |
Pro Forma Weighted Average Shares Outstanding, Diluted |  |  |  | 53.1 |

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- Definition Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275 + Details
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- Definition With respect to a business combination completed during the period, this element provides a description of the business, other than the name, which may include the industry, size, products and other important information. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
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- Definition The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
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