Selected Quarterly Financial Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net sales	$ 3,838.6	$ 4,033.9	$ 3,994.4	$ 3,324.7	$ 3,492.4	$ 3,708.2	$ 3,664.6	$ 3,116.7	$ 15,191.5	$ 13,981.9	$ 12,988.7
Gross profit	614.4	642.0	641.1	552.6	577.9	614.3	610.5	524.5	2,449.9	2,327.2	2,115.8
Income (loss) from operations	221.6	243.7	231.1	169.8	197.2	237.5	223.5	161.0	866.1	819.2	742.0
Net income	$ 195.2	$ 129.2	$ 141.0	$ 57.6	$ 103.2	$ 125.9	$ 117.5	$ 77.8	$ 523.0	$ 424.4	$ 403.1
Earnings per share, basic (in dollars per share)	$ 1.28	$ 0.84	$ 0.90	$ 0.36	$ 0.64	$ 0.78	$ 0.71	$ 0.47	$ 3.37	$ 2.59	$ 2.37
Earnings per share, diluted (in dollars per share)	1.26	0.83	0.89	0.35	0.63	0.76	0.70	0.46	3.31	2.56	2.35
Cash dividends declared per common share (in dollars per share)	$ 0.2100	$ 0.1600	$ 0.1600	$ 0.1600	$ 0.1600	$ 0.1075	$ 0.1075	$ 0.1075	$ 0.6900	$ 0.4825	$ 0.3100
Corporate
Segment Reporting Information [Line Items]
Net sales	$ 1,641.4	$ 1,598.5	$ 1,630.7	$ 1,476.3	$ 1,517.7	$ 1,466.4	$ 1,490.8	$ 1,414.9
Small Business
Segment Reporting Information [Line Items]
Net sales	314.8	311.5	321.5	298.7	291.8	282.5	288.4	277.4
Public
Segment Reporting Information [Line Items]
Net sales	1,454.1	1,732.9	1,674.1	1,176.5	1,332.3	1,640.6	1,547.0	1,069.4
Public | Government
Segment Reporting Information [Line Items]
Net sales	630.0	606.7	543.9	386.9	529.6	537.5	456.6	339.9
Public | Education
Segment Reporting Information [Line Items]
Net sales	400.8	700.7	712.9	397.1	365.9	671.4	640.0	341.0
Public | Healthcare
Segment Reporting Information [Line Items]
Net sales	423.3	425.5	417.3	392.5	436.8	431.7	450.4	388.5
Other
Segment Reporting Information [Line Items]
Net sales	$ 428.3	$ 391.0	$ 368.1	$ 373.2	$ 350.5	$ 318.7	$ 338.4	$ 355.0