Selected Quarterly Financial Results (unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Gross profit	777.5	816.5	773.8	672.1	693.8	713.6	695.6	603.9	3,039.9	2,706.9	2,450.2
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
Net income	$ 185.6	$ 201.7	$ 196.6	$ 152.9	$ 159.3	$ 183.7	$ 173.0	$ 127.0	$ 736.8	$ 643.0	$ 523.1
Earnings per share, basic (in dollars per share)	$ 1.29	$ 1.39	$ 1.35	$ 1.04	$ 1.07	$ 1.22	$ 1.14	$ 0.83	$ 5.08	$ 4.26	$ 3.37
Earnings per share, diluted (in dollars per share)	$ 1.27	$ 1.37	$ 1.33	$ 1.02	$ 1.05	$ 1.20	$ 1.12	$ 0.82	$ 4.99	$ 4.19	$ 3.31
Corporate
Segment Reporting Information [Line Items]
Net sales	$ 1,965.4	$ 1,913.5	$ 1,883.9	$ 1,736.2	$ 1,836.4	$ 1,706.5	$ 1,733.8	$ 1,565.8
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
Public
Segment Reporting Information [Line Items]
Net sales	1,593.1	2,100.9	1,840.1	1,330.7	1,414.2	1,875.1	1,635.4	1,230.0
Public | Government
Segment Reporting Information [Line Items]
Net sales	659.1	793.4	578.4	488.4	546.0	639.3	493.5	418.5
Public | Education
Segment Reporting Information [Line Items]
Net sales	430.6	807.0	773.6	400.4	425.0	793.1	712.1	397.2
Public | Healthcare
Segment Reporting Information [Line Items]
Net sales	503.4	500.5	488.1	441.9	443.2	442.7	429.8	414.3
Other
Segment Reporting Information [Line Items]
Net sales	$ 587.3	$ 507.1	$ 528.5	$ 535.4	$ 461.7	$ 451.6	$ 487.4	$ 483.0