Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Net sales	$ 5,949.1	$ 4,837.5
Percentage of Total Net Sales	100.00%	100.00%
Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 2,627.6	$ 1,805.6
Small Business
Disaggregation of Revenue [Line Items]
Net sales	524.0	432.7
Public
Disaggregation of Revenue [Line Items]
Net sales	2,033.0	1,921.7
Other
Disaggregation of Revenue [Line Items]
Net sales	764.5	677.5
Transferred at a point in time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales	5,288.6	4,456.2
Transferred at a point in time where CDW is principal | Corporate
Disaggregation of Revenue [Line Items]
Net sales	2,291.2	1,627.7
Transferred at a point in time where CDW is principal | Small Business
Disaggregation of Revenue [Line Items]
Net sales	480.0	402.0
Transferred at a point in time where CDW is principal | Public
Disaggregation of Revenue [Line Items]
Net sales	1,840.3	1,825.9
Transferred at a point in time where CDW is principal | Other
Disaggregation of Revenue [Line Items]
Net sales	677.1	600.6
Transferred at a point in time where CDW is agent
Disaggregation of Revenue [Line Items]
Net sales	340.7	220.7
Transferred at a point in time where CDW is agent | Corporate
Disaggregation of Revenue [Line Items]
Net sales	180.6	112.3
Transferred at a point in time where CDW is agent | Small Business
Disaggregation of Revenue [Line Items]
Net sales	34.3	26.9
Transferred at a point in time where CDW is agent | Public
Disaggregation of Revenue [Line Items]
Net sales	96.3	58.1
Transferred at a point in time where CDW is agent | Other
Disaggregation of Revenue [Line Items]
Net sales	29.5	23.4
Transferred over time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales	319.8	160.6
Transferred over time where CDW is principal | Corporate
Disaggregation of Revenue [Line Items]
Net sales	155.8	65.6
Transferred over time where CDW is principal | Small Business
Disaggregation of Revenue [Line Items]
Net sales	9.7	3.8
Transferred over time where CDW is principal | Public
Disaggregation of Revenue [Line Items]
Net sales	96.4	37.7
Transferred over time where CDW is principal | Other
Disaggregation of Revenue [Line Items]
Net sales	57.9	53.5
Corporate
Disaggregation of Revenue [Line Items]
Net sales	2,627.6	1,805.6
Corporate | Corporate
Disaggregation of Revenue [Line Items]
Net sales	2,627.6	1,805.6
Corporate | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Corporate | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Corporate | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Small Business
Disaggregation of Revenue [Line Items]
Net sales	524.0	432.7
Small Business | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Small Business | Small Business
Disaggregation of Revenue [Line Items]
Net sales	524.0	432.7
Small Business | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Small Business | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Government
Disaggregation of Revenue [Line Items]
Net sales	543.9	516.1
Government | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Government | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Government | Public
Disaggregation of Revenue [Line Items]
Net sales	543.9	516.1
Government | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Education
Disaggregation of Revenue [Line Items]
Net sales	902.8	943.3
Education | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Education | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Education | Public
Disaggregation of Revenue [Line Items]
Net sales	902.8	943.3
Education | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Healthcare
Disaggregation of Revenue [Line Items]
Net sales	586.3	462.3
Healthcare | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Healthcare | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Healthcare | Public
Disaggregation of Revenue [Line Items]
Net sales	586.3	462.3
Healthcare | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Net sales	764.5	677.5
Other | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Other | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Other | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Other | Other
Disaggregation of Revenue [Line Items]
Net sales	764.5	677.5
Hardware
Disaggregation of Revenue [Line Items]
Net sales	$ 4,682.1	$ 3,965.2
Percentage of Total Net Sales	78.70%	82.00%
Hardware | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 1,983.6	$ 1,420.6
Hardware | Small Business
Disaggregation of Revenue [Line Items]
Net sales	443.5	368.1
Hardware | Public
Disaggregation of Revenue [Line Items]
Net sales	1,646.3	1,664.9
Hardware | Other
Disaggregation of Revenue [Line Items]
Net sales	608.7	511.6
Notebooks/Mobile Devices
Disaggregation of Revenue [Line Items]
Net sales	$ 1,713.8	$ 1,591.8
Percentage of Total Net Sales	28.80%	32.90%
Netcomm Products
Disaggregation of Revenue [Line Items]
Net sales	$ 486.4	$ 454.7
Percentage of Total Net Sales	8.20%	9.40%
Desktops
Disaggregation of Revenue [Line Items]
Net sales	$ 337.5	$ 265.3
Percentage of Total Net Sales	5.70%	5.50%
Video
Disaggregation of Revenue [Line Items]
Net sales	$ 485.9	$ 326.2
Percentage of Total Net Sales	8.20%	6.70%
Enterprise and Data Storage (Including Drives)
Disaggregation of Revenue [Line Items]
Net sales	$ 353.7	$ 259.9
Percentage of Total Net Sales	5.90%	5.40%
Other Hardware
Disaggregation of Revenue [Line Items]
Net sales	$ 1,304.8	$ 1,067.3
Percentage of Total Net Sales	21.90%	22.10%
Software
Disaggregation of Revenue [Line Items]
Net sales	$ 798.4	$ 610.0
Percentage of Total Net Sales	13.40%	12.60%
Software | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 399.8	$ 267.3
Software | Small Business
Disaggregation of Revenue [Line Items]
Net sales	59.8	49.2
Software | Public
Disaggregation of Revenue [Line Items]
Net sales	253.2	194.4
Software | Other
Disaggregation of Revenue [Line Items]
Net sales	85.6	99.1
Services
Disaggregation of Revenue [Line Items]
Net sales	$ 437.5	$ 233.7
Percentage of Total Net Sales	7.40%	4.80%
Services | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 225.3	$ 102.5
Services | Small Business
Disaggregation of Revenue [Line Items]
Net sales	15.1	10.1
Services | Public
Disaggregation of Revenue [Line Items]
Net sales	129.2	57.9
Services | Other
Disaggregation of Revenue [Line Items]
Net sales	67.9	63.2
Other
Disaggregation of Revenue [Line Items]
Net sales	$ 31.1	$ 28.6
Percentage of Total Net Sales	0.50%	0.60%
Other | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 18.9	$ 15.2
Other | Small Business
Disaggregation of Revenue [Line Items]
Net sales	5.6	5.3
Other | Public
Disaggregation of Revenue [Line Items]
Net sales	4.3	4.5
Other | Other
Disaggregation of Revenue [Line Items]
Net sales	2.3	3.6
United States
Disaggregation of Revenue [Line Items]
Net sales	5,178.4	4,162.5
United States | Corporate
Disaggregation of Revenue [Line Items]
Net sales	2,616.8	1,803.5
United States | Small Business
Disaggregation of Revenue [Line Items]
Net sales	524.0	432.7
United States | Public
Disaggregation of Revenue [Line Items]
Net sales	2,033.0	1,921.7
United States | Other
Disaggregation of Revenue [Line Items]
Net sales	4.6	4.6
Rest of World
Disaggregation of Revenue [Line Items]
Net sales	770.7	675.0
Rest of World | Corporate
Disaggregation of Revenue [Line Items]
Net sales	10.8	2.1
Rest of World | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Rest of World | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Rest of World | Other
Disaggregation of Revenue [Line Items]
Net sales	$ 759.9	$ 672.9