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Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net sales	$ 4,536.9	$ 4,907.7	$ 4,629.9	$ 3,957.9	$ 4,074.8	$ 4,373.2	$ 4,186.1	$ 3,606.4	$ 18,032.4	$ 16,240.5	$ 14,832.9
Income (loss) from operations	283.8	320.6	300.3	228.9	242.9	274.8	265.5	204.1	1,133.6	987.3	866.5
Depreciation and amortization expense									(267.1)	(265.6)	(260.9)
Small Business
Segment Reporting Information [Line Items]
Net sales	391.1	386.2	377.4	355.6	362.5	340.0	329.5	327.6
Other
Segment Reporting Information [Line Items]
Net sales	$ 587.3	$ 507.1	$ 528.5	$ 535.4	$ 461.7	$ 451.6	$ 487.4	$ 483.0
Operating Segments \| Corporate
Segment Reporting Information [Line Items]
Net sales									7,499.0	6,842.5	6,172.8
Income (loss) from operations									585.1	530.4	480.9
Depreciation and amortization expense									(86.9)	(88.2)	(90.1)
Operating Segments \| Small Business
Segment Reporting Information [Line Items]
Net sales									1,510.3	1,359.6	1,220.5
Income (loss) from operations									107.5	94.4	73.0
Depreciation and amortization expense									(22.5)	(22.1)	(22.0)
Operating Segments \| Public
Segment Reporting Information [Line Items]
Net sales									6,864.8	6,154.7	5,906.5
Income (loss) from operations									475.0	405.0	367.7
Depreciation and amortization expense									(56.3)	(51.2)	(51.5)
Operating Segments \| Other
Segment Reporting Information [Line Items]
Net sales									2,158.3	1,883.7	1,533.1
Income (loss) from operations									101.6	82.2	57.1
Depreciation and amortization expense									(31.2)	(31.8)	(30.9)
Headquarters
Segment Reporting Information [Line Items]
Net sales									0.0	0.0	0.0
Income (loss) from operations									(135.6)	(124.7)	(112.2)
Depreciation and amortization expense									$ (70.2)	$ (72.3)	$ (66.4)