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