Goodwill and Identifiable Assets (Goodwill) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Gross goodwill	$ 1,475.5	$ 1,420.7	$ 1,291.3
Goodwill, Purchase Accounting Adjustments	3.5
Translation adjustment		(0.1)
Business Acquisition, Goodwill, Expected Tax Deductible Amount	51.3
Federal Government Business
Goodwill [Roll Forward]
Gross goodwill	645.6	642.1	552.9
Goodwill, Purchase Accounting Adjustments	3.5
Commercial Business
Goodwill [Roll Forward]
Gross goodwill	829.9	778.6	$ 738.4
Translation adjustment		(0.1)
2020 Acquisitions
Goodwill [Roll Forward]
Goodwill, Acquired During Period		129.5
2020 Acquisitions | Federal Government Business
Goodwill [Roll Forward]
Goodwill, Acquired During Period		89.2
2020 Acquisitions | Commercial Business
Goodwill [Roll Forward]
Goodwill, Acquired During Period		40.3
2021 Acquisitions
Goodwill [Roll Forward]
Business Acquisition, Goodwill, Expected Tax Deductible Amount	47.2	$ 77.1
2021 Acquisitions | Commercial Business
Goodwill [Roll Forward]
Business Acquisition, Goodwill, Expected Tax Deductible Amount	51.3
Accumulated Acquisition since 2019
Goodwill [Roll Forward]
Business Acquisition, Goodwill, Expected Tax Deductible Amount	$ 124.3