Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Net sales	$ 4,907.7	$ 4,373.2	$ 13,495.5	$ 12,165.7
Income (loss) from operations	320.6	274.8	849.8	744.4
Depreciation and amortization expense	(68.2)	(65.8)	(197.5)	(198.7)
Operating Segments | Corporate
Segment Reporting Information [Line Items]
Net sales	1,913.5	1,706.5	5,533.6	5,006.1
Income (loss) from operations	138.9	125.7	430.8	388.3
Depreciation and amortization expense	(21.7)	(21.8)	(64.8)	(66.8)
Operating Segments | Small Business
Segment Reporting Information [Line Items]
Net sales	386.2	340.0	1,119.2	997.1
Income (loss) from operations	27.2	23.8	78.2	70.0
Depreciation and amortization expense	(5.6)	(5.5)	(16.8)	(16.6)
Operating Segments | Public
Segment Reporting Information [Line Items]
Net sales	2,100.9	1,875.1	5,271.7	4,740.5
Income (loss) from operations	168.4	136.6	373.9	318.3
Depreciation and amortization expense	(15.0)	(12.9)	(41.4)	(38.4)
Operating Segments | Other
Segment Reporting Information [Line Items]
Net sales	507.1	451.6	1,571.0	1,422.0
Income (loss) from operations	21.9	19.2	66.6	60.7
Depreciation and amortization expense	(7.9)	(7.3)	(23.1)	(25.1)
Headquarters
Segment Reporting Information [Line Items]
Net sales	0.0	0.0	0.0	0.0
Income (loss) from operations	(35.8)	(30.5)	(99.7)	(92.9)
Depreciation and amortization expense	$ (18.0)	$ (18.3)	$ (51.4)	$ (51.8)