Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net sales	$ 4,373.2	$ 3,933.2	$ 12,165.7	$ 11,080.8
Income (loss) from operations	274.8	244.0	744.4	645.6
Depreciation and amortization expense	(65.8)	(65.7)	(198.7)	(195.2)
Operating Segments | Corporate
Segment Reporting Information [Line Items]
Net sales	1,706.5	1,552.8	5,006.1	4,573.5
Income (loss) from operations	127.2	121.6	393.3	360.6
Depreciation and amortization expense	(20.3)	(20.8)	(61.8)	(62.5)
Operating Segments | Small Business
Segment Reporting Information [Line Items]
Net sales	340.0	305.4	997.1	912.4
Income (loss) from operations	24.1	17.7	71.0	53.7
Depreciation and amortization expense	(5.2)	(5.2)	(15.6)	(15.4)
Operating Segments | Public
Segment Reporting Information [Line Items]
Net sales	1,875.1	1,693.9	4,740.5	4,480.4
Income (loss) from operations	138.0	122.3	322.7	287.6
Depreciation and amortization expense	(11.5)	(11.2)	(34.0)	(33.6)
Operating Segments | Other
Segment Reporting Information [Line Items]
Net sales	451.6	381.1	1,422.0	1,114.5
Income (loss) from operations	19.2	14.2	60.7	39.5
Depreciation and amortization expense	(7.3)	(8.0)	(25.1)	(22.7)
Headquarters
Segment Reporting Information [Line Items]
Net sales	0.0	0.0	0.0	0.0
Income (loss) from operations	(33.7)	(31.8)	(103.3)	(95.8)
Depreciation and amortization expense	$ (21.5)	$ (20.5)	$ (62.2)	$ (61.0)