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Acquisitions Business Combinations Additional Information (Details) - USD ($) $ in Millions |  |  |  | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Aug. 31, 2020 | Jan. 24, 2020 | Oct. 17, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019
Acquired Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Payments to Acquire Businesses, Net of Cash Acquired |  |  |  | $ 151.5 | $ 48.5 |
Contingent purchase consideration |  |  |  | 6.0 |  | $ 0.0
Business Acquisition, Description of Acquired Entity | headquartered in Boston, Massachusetts |  |  |  |  |
Business Acquisition, Goodwill, Expected Tax Deductible Amount |  |  | $ 38.7 |  |  |
LeapFrog |  |  |  |  |  |
Acquired Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Business Acquisition, Effective Date of Acquisition | Aug. 31, 2020 |  |  |  |  |
Business Acquisition, Name of Acquired Entity | LeapFrog Systems, Inc. |  |  |  |  |
Payments to Acquire Businesses, Net of Cash Acquired | $ 66.0 |  |  |  |  |
Contingent purchase consideration | $ 6.0 |  |  |  |  |
Business Combination, Reason for Business Combination | acquisition deepens and expands the Company's capabilities in digital innovation and enterprise solutions for its financial services, insurance and healthcare clients |  |  |  |  |
Goodwill, Acquired During Period | $ 42.4 |  |  | 42.4 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets | $ 35.1 |  |  |  |  |
BlackStone Federal |  |  |  |  |  |
Acquired Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Business Acquisition, Effective Date of Acquisition |  | Jan. 24, 2020 |  |  |  |
Business Acquisition, Name of Acquired Entity |  | Blackstone Technology Group |  |  |  |
Payments to Acquire Businesses, Net of Cash Acquired |  | $ 85.5 |  |  |  |
Business Acquisition, Description of Acquired Entity |  | headquartered in Arlington, Virginia |  |  |  |
Business Combination, Reason for Business Combination |  | The acquisition expands the Company's capabilities in agile application development, cloud modernization and systems architecture, cybersecurity, user experience design and branding services to government clients |  |  |  |
Goodwill, Acquired During Period |  | $ 61.1 |  | $ 61.1 |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets |  | $ 22.8 |  |  |  |
Intersys |  |  |  |  |  |
Acquired Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Business Acquisition, Effective Date of Acquisition |  |  | Oct. 17, 2019 |  |  |
Business Acquisition, Name of Acquired Entity |  |  | Intersys Consulting, LLC |  |  |
Payments to Acquire Businesses, Net of Cash Acquired |  |  | $ 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 Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets |  |  | $ 23.8 |  |  |
Minimum [Member] | LeapFrog |  |  |  |  |  |
Acquired Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Finite-Lived Intangible Asset, Useful Life | 2 years |  |  |  |  |
Minimum [Member] | BlackStone Federal |  |  |  |  |  |
Acquired Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Finite-Lived Intangible Asset, Useful Life |  | 1 year |  |  |  |
Minimum [Member] | Intersys |  |  |  |  |  |
Acquired Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Finite-Lived Intangible Asset, Useful Life |  |  | 3 years |  |  |
Maximum [Member] | LeapFrog |  |  |  |  |  |
Acquired Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Finite-Lived Intangible Asset, Useful Life | 7 years |  |  |  |  |
Maximum [Member] | BlackStone Federal |  |  |  |  |  |
Acquired Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Finite-Lived Intangible Asset, Useful Life |  | 9 years |  |  |  |
Maximum [Member] | Intersys |  |  |  |  |  |
Acquired Indefinite-lived Intangible Assets [Line Items] |  |  |  |  |  |
Finite-Lived Intangible Asset, Useful Life |  |  | 10 years |  |  |

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