Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Net sales	$ 4,389.2	$ 3,957.9
Percentage of Total Net Sales	100.00%	100.00%
Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 1,911.0	$ 1,736.2
Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	391.5	355.6
Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,525.3	1,330.7
Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	561.4	535.4
Transferred at a point in time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales	4,011.3	3,635.2
Transferred at a point in time where CDW is principal | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,741.0	1,581.8
Transferred at a point in time where CDW is principal | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	366.7	335.2
Transferred at a point in time where CDW is principal | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,404.6	1,243.3
Transferred at a point in time where CDW is principal | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	499.0	474.9
Transferred at a point in time where CDW is agent
Disaggregation of Revenue [Line Items]
Net sales	195.1	183.0
Transferred at a point in time where CDW is agent | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	107.6	104.0
Transferred at a point in time where CDW is agent | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	22.0	19.8
Transferred at a point in time where CDW is agent | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	52.3	46.2
Transferred at a point in time where CDW is agent | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	13.2	13.0
Transferred over time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales	182.8	139.7
Transferred over time where CDW is principal | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	62.4	50.4
Transferred over time where CDW is principal | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	2.8	0.6
Transferred over time where CDW is principal | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	68.4	41.2
Transferred over time where CDW is principal | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	49.2	47.5
Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,911.0	1,736.2
Corporate | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,911.0	1,736.2
Corporate | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Corporate | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Corporate | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Small Business
Disaggregation of Revenue [Line Items]
Net sales	391.5	355.6
Small Business | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Small Business | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	391.5	355.6
Small Business | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Small Business | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Government
Disaggregation of Revenue [Line Items]
Net sales	568.5	488.4
Government | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Government | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Government | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	568.5	488.4
Government | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Education
Disaggregation of Revenue [Line Items]
Net sales	476.2	400.4
Education | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Education | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Education | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	476.2	400.4
Education | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Healthcare
Disaggregation of Revenue [Line Items]
Net sales	480.6	441.9
Healthcare | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Healthcare | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Healthcare | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	480.6	441.9
Healthcare | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Net sales	561.4	535.4
Other | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Other | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Other | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Other | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	561.4	535.4
Hardware
Disaggregation of Revenue [Line Items]
Net sales	$ 3,456.5	$ 3,146.3
Percentage of Total Net Sales	78.70%	79.50%
Hardware | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 1,513.9	$ 1,384.7
Hardware | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	324.4	295.1
Hardware | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,210.8	1,063.3
Hardware | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	407.4	403.2
Notebooks/Mobile Devices
Disaggregation of Revenue [Line Items]
Net sales	$ 1,111.0	$ 916.1
Percentage of Total Net Sales	25.30%	23.10%
Netcomm Products
Disaggregation of Revenue [Line Items]
Net sales	$ 456.4	$ 511.4
Percentage of Total Net Sales	10.40%	12.90%
Desktops
Disaggregation of Revenue [Line Items]
Net sales	$ 384.3	$ 297.0
Percentage of Total Net Sales	8.80%	7.50%
Video
Disaggregation of Revenue [Line Items]
Net sales	$ 314.6	$ 271.3
Percentage of Total Net Sales	7.20%	6.90%
Enterprise and Data Storage (Including Drives)
Disaggregation of Revenue [Line Items]
Net sales	$ 240.5	$ 293.3
Percentage of Total Net Sales	5.50%	7.40%
Other Hardware
Disaggregation of Revenue [Line Items]
Net sales	$ 949.7	$ 857.2
Percentage of Total Net Sales	21.50%	21.70%
Software
Disaggregation of Revenue [Line Items]
Net sales	$ 666.3	$ 593.0
Percentage of Total Net Sales	15.20%	15.00%
Software | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 276.0	$ 246.5
Software | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	53.5	48.1
Software | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	239.7	221.1
Software | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	97.1	77.3
Services
Disaggregation of Revenue [Line Items]
Net sales	$ 230.6	$ 189.0
Percentage of Total Net Sales	5.30%	4.80%
Services | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 100.1	$ 88.0
Services | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	8.2	7.0
Services | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	67.8	42.1
Services | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	54.5	51.9
Other
Disaggregation of Revenue [Line Items]
Net sales	$ 35.8	$ 29.6
Percentage of Total Net Sales	0.80%	0.70%
Other | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 21.0	$ 17.0
Other | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	5.4	5.4
Other | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	7.0	4.2
Other | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	2.4	3.0
United States
Disaggregation of Revenue [Line Items]
Net sales	3,830.8	3,427.6
United States | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,907.8	1,732.9
United States | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	391.5	355.6
United States | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,525.3	1,330.7
United States | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	6.2	8.4
Rest of World
Disaggregation of Revenue [Line Items]
Net sales	558.4	530.3
Rest of World | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	3.2	3.3
Rest of World | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Rest of World | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Rest of World | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	$ 555.2	$ 527.0