Selected Quarterly Financial Results (unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Gross profit	693.8	713.6	695.6	603.9	613.7	642.2	640.8	553.5	2,706.9	2,450.2	2,328.3
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
Net income	$ 159.3	$ 183.7	$ 173.0	$ 127.0	$ 194.8	$ 129.3	$ 140.9	$ 58.2	$ 643.0	$ 523.1	$ 425.1
Earnings per share, basic (in dollars per share)	$ 1.07	$ 1.22	$ 1.14	$ 0.83	$ 1.28	$ 0.84	$ 0.90	$ 0.36	$ 4.26	$ 3.37	$ 2.60
Earnings per share, diluted (in dollars per share)	1.05	1.20	1.12	0.82	1.25	0.83	0.89	0.36	4.19	3.31	2.56
Cash dividends declared per common share (in dollars per share)	$ 0.295	$ 0.210	$ 0.210	$ 0.210	$ 0.210	$ 0.160	$ 0.160	$ 0.160	$ 0.9250	$ 0.6900	$ 0.4825
Corporate
Segment Reporting Information [Line Items]
Net sales	$ 1,836.4	$ 1,706.5	$ 1,733.8	$ 1,565.8	$ 1,599.3	$ 1,552.8	$ 1,580.1	$ 1,440.6
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
Public
Segment Reporting Information [Line Items]
Net sales	1,414.2	1,875.1	1,635.4	1,230.0	1,426.1	1,693.9	1,632.8	1,153.7
Public | Government
Segment Reporting Information [Line Items]
Net sales	546.0	639.3	493.5	418.5	619.9	591.9	523.4	374.6
Public | Education
Segment Reporting Information [Line Items]
Net sales	425.0	793.1	712.1	397.2	395.2	691.3	704.9	393.2
Public | Healthcare
Segment Reporting Information [Line Items]
Net sales	443.2	442.7	429.8	414.3	411.0	410.7	404.5	385.9
Other
Segment Reporting Information [Line Items]
Net sales	$ 461.7	$ 451.6	$ 487.4	$ 483.0	$ 418.6	$ 381.1	$ 363.8	$ 369.7