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Amount before 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.

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Goodwill and Identifiable Assets (Goodwill) (Details) - USD ($) $ in Millions				9 Months Ended	12 Months Ended
	Aug. 31, 2020	Jan. 24, 2020	Oct. 17, 2019	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Gross goodwill				$ 1,590.9	$ 1,486.9	$ 1,421.1
Translation adjustment				0.5	(0.3)
Apex
Goodwill [Roll Forward]
Gross goodwill				745.5	703.5	662.1
Translation adjustment				(0.4)
Oxford
Goodwill [Roll Forward]
Gross goodwill				231.4	230.5	230.8
Translation adjustment				0.9	(0.3)
ECS
Goodwill [Roll Forward]
Gross goodwill				614.0	552.9	$ 528.2
DHA
Goodwill [Roll Forward]
Goodwill, Acquired During Period					24.7
Intersys
Goodwill [Roll Forward]
Goodwill, Acquired During Period			$ 41.4		$ 41.4
BlackStone Federal
Goodwill [Roll Forward]
Goodwill, Acquired During Period		$ 61.1		61.1
LeapFrog
Goodwill [Roll Forward]
Goodwill, Acquired During Period	$ 42.4			$ 42.4