Selected Quarterly Financial Results (unaudited) (Details) - USD ($) $ / shares in Units, $ 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]
Net sales	$ 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
Gross profit	880.9	825.5	747.2	756.5	777.5	816.5	773.8	672.1	3,210.1	3,039.9	2,706.9
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
Net income	$ 238.3	$ 193.2	$ 189.1	$ 167.9	$ 185.6	$ 201.7	$ 196.6	$ 152.9	$ 788.5	$ 736.8	$ 643.0
Basic (in dollars per share)	$ 1.67	$ 1.36	$ 1.32	$ 1.18	$ 1.29	$ 1.39	$ 1.35	$ 1.04	$ 5.53	$ 5.08	$ 4.26
Diluted (in dollars per share)	$ 1.65	$ 1.33	$ 1.31	$ 1.16	$ 1.27	$ 1.37	$ 1.33	$ 1.02	$ 5.45	$ 4.99	$ 4.19
Corporate
Segment Reporting Information [Line Items]
Net sales	$ 1,717.5	$ 1,660.0	$ 1,557.5	$ 1,911.0	$ 1,965.4	$ 1,913.5	$ 1,883.9	$ 1,736.2
Small Business
Segment Reporting Information [Line Items]
Net sales	366.5	337.0	302.1	391.5	391.1	386.2	377.4	355.6
Public
Segment Reporting Information [Line Items]
Net sales	2,296.5	2,293.8	2,022.1	1,525.3	1,593.1	2,100.9	1,840.1	1,330.7
Public | Government
Segment Reporting Information [Line Items]
Net sales	842.6	847.7	719.7	568.5	659.1	793.4	578.4	488.4
Public | Education
Segment Reporting Information [Line Items]
Net sales	1,026.9	1,078.2	876.8	476.2	430.6	807.0	773.6	400.4
Public | Healthcare
Segment Reporting Information [Line Items]
Net sales	427.0	367.9	425.6	480.6	503.4	500.5	488.1	441.9
Other
Segment Reporting Information [Line Items]
Net sales	$ 575.7	$ 465.6	$ 484.0	$ 561.4	$ 587.3	$ 507.1	$ 528.5	$ 535.4