Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 4,956.2	$ 4,756.4	$ 4,365.7	$ 4,389.2	$ 4,536.9	$ 4,907.7	$ 4,629.9	$ 3,957.9	$ 18,467.5	$ 18,032.4	$ 16,240.5
Income (loss) from operations	332.2	317.8	283.4	245.8	283.8	320.6	300.3	228.9	1,179.2	1,133.6	987.3
Depreciation and amortization expense									(425.6)	(267.1)	(265.6)
Small Business
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	366.5	337.0	302.1	391.5	391.1	386.2	377.4	355.6
Other
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 575.7	$ 465.6	$ 484.0	$ 561.4	$ 587.3	$ 507.1	$ 528.5	$ 535.4
Operating Segments | Corporate
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									6,846.0	7,499.0	6,842.5
Income (loss) from operations									489.5	585.1	530.4
Depreciation and amortization expense									(73.2)	(86.9)	(88.2)
Operating Segments | Small Business
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,397.1	1,510.3	1,359.6
Income (loss) from operations									99.0	107.5	94.4
Depreciation and amortization expense									(18.3)	(22.5)	(22.1)
Operating Segments | Public
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									8,137.7	6,864.8	6,154.7
Income (loss) from operations									678.2	475.0	405.0
Depreciation and amortization expense									(229.7)	(56.3)	(51.2)
Operating Segments | Other
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									2,086.7	2,158.3	1,883.7
Income (loss) from operations									65.9	101.6	82.2
Depreciation and amortization expense									(32.5)	(31.2)	(31.8)
Headquarters
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Income (loss) from operations									(153.4)	(135.6)	(124.7)
Depreciation and amortization expense									$ (71.9)	$ (70.2)	$ (72.3)