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Acquisitions Business Combinations Additional Information (Details) - USD ($) $ in Millions |  |  | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 24, 2020 | Oct. 17, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Dec. 31, 2019
Business Acquisition [Line Items] |  |  |  |  |
Cash paid for acquisitions, net of cash acquired |  |  | $ 85.5 | $ 48.5 |
Goodwill |  |  | 1,547.3 |  | $ 1,486.9
BlackStone Federal |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business Acquisition, Effective Date of Acquisition | Jan. 24, 2020 |  |  |  |
Business Acquisition, Name of Acquired Entity | Blackstone Technology Group |  |  |  |
Cash paid for acquisitions, 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 |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business Acquisition, Effective Date of Acquisition |  | Oct. 17, 2019 |  |  |
Business Acquisition, Name of Acquired Entity |  | Intersys Consulting, LLC |  |  |
Cash paid for acquisitions, 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 |  | $ 41.4 |  |  |
Goodwill, Acquired During Period |  |  |  |  | $ 41.4
Business Acquisition, Goodwill, Expected Tax Deductible Amount |  | 38.7 |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets |  | $ 23.8 |  |  |
Minimum [Member] | BlackStone Federal |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Finite-Lived Intangible Asset, Useful Life | 1 year |  |  |  |
Minimum [Member] | Intersys |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Finite-Lived Intangible Asset, Useful Life |  | 3 years |  |  |
Maximum [Member] | BlackStone Federal |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Finite-Lived Intangible Asset, Useful Life | 9 years |  |  |  |
Maximum [Member] | Intersys |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Finite-Lived Intangible Asset, Useful Life |  | 10 years |  |  |

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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 Date when the acquirer obtains control of the acquiree, in CCYY-MM-DD format. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=116868678&loc=d3e848-128460 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=116868678&loc=d3e845-128460 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
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- Definition Name of the acquired entity. + 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 amount of goodwill arising from a business combination that is expected to be deductible for tax purposes. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479 + Details
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- Definition This element represents a description of the primary reason for the business combination which may consist of general categories such as top-line growth, synergistic benefits, market share, and diversification and the more detailed factors that might apply. + 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 (d) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
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- Definition Amount of assets, excluding financial assets and goodwill, that lack physical substance, having a projected indefinite period of benefit, acquired at the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121647850&loc=d3e4845-128472 + Details
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- Definition Amount after 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 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(10)(1)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
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