Segment Information - Schedule of Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net sales	$ 4,074.8	$ 4,373.2	$ 4,186.1	$ 3,606.4	$ 3,752.1	$ 3,933.2	$ 3,891.7	$ 3,256.0	$ 16,240.5	$ 14,832.9	$ 13,672.7
Income (loss) from operations	242.9	274.8	265.5	204.1	221.1	244.0	230.8	170.7	987.3	866.5	820.0
Depreciation and amortization expense									(265.6)	(260.9)	(254.5)
Small Business
Segment Reporting Information [Line Items]
Net sales	362.5	340.0	329.5	327.6	308.1	305.4	315.0	292.0
Other
Segment Reporting Information [Line Items]
Net sales	$ 461.7	$ 451.6	$ 487.4	$ 483.0	$ 418.6	$ 381.1	$ 363.8	$ 369.7
Operating Segments | Corporate
Segment Reporting Information [Line Items]
Net sales									6,842.5	6,172.8	5,734.9
Income (loss) from operations									536.9	487.9	453.5
Depreciation and amortization expense									(81.7)	(83.1)	(82.9)
Operating Segments | Small Business
Segment Reporting Information [Line Items]
Net sales									1,359.6	1,220.5	1,118.1
Income (loss) from operations									95.7	74.3	69.1
Depreciation and amortization expense									(20.8)	(20.7)	(20.6)
Operating Segments | Public
Segment Reporting Information [Line Items]
Net sales									6,154.7	5,906.5	5,477.4
Income (loss) from operations									410.8	374.4	367.7
Depreciation and amortization expense									(45.4)	(44.8)	(44.7)
Operating Segments | Other
Segment Reporting Information [Line Items]
Net sales									1,883.7	1,533.1	1,342.3
Income (loss) from operations									82.2	57.1	44.6
Depreciation and amortization expense									(31.8)	(30.9)	(32.1)
Headquarters
Segment Reporting Information [Line Items]
Net sales									0.0	0.0	0.0
Income (loss) from operations									(138.3)	(127.2)	(114.9)
Depreciation and amortization expense									$ (85.9)	$ (81.4)	$ (74.2)