Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Net sales	$ 3,606.4	$ 3,256.0
Revenue net, by Product and Service, Percentage	100.00%	100.00%
Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 1,565.8	$ 1,440.6
Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	327.6	292.0
Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,230.0	1,153.7
Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	483.0	369.7
Transferred at a point in time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales	3,340.1	3,027.2
Transferred at a point in time where CDW is principal | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,426.3	1,314.2
Transferred at a point in time where CDW is principal | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	309.4	275.7
Transferred at a point in time where CDW is principal | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,171.4	1,105.8
Transferred at a point in time where CDW is principal | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	433.0	331.5
Transferred at a point in time where CDW is agent
Disaggregation of Revenue [Line Items]
Net sales	165.4	141.0
Transferred at a point in time where CDW is agent | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	92.7	81.3
Transferred at a point in time where CDW is agent | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	16.5	14.0
Transferred at a point in time where CDW is agent | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	41.1	37.5
Transferred at a point in time where CDW is agent | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	15.1	8.2
Transferred over time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales	100.9	87.8
Transferred over time where CDW is principal | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	46.8	45.1
Transferred over time where CDW is principal | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	1.7	2.3
Transferred over time where CDW is principal | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	17.5	10.4
Transferred over time where CDW is principal | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	34.9	30.0
Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,565.8	1,440.6
Corporate | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,565.8	1,440.6
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	327.6	292.0
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	327.6	292.0
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	418.5	374.6
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	418.5	374.6
Government | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Education
Disaggregation of Revenue [Line Items]
Net sales	397.2	393.2
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	397.2	393.2
Education | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Healthcare
Disaggregation of Revenue [Line Items]
Net sales	414.3	385.9
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	414.3	385.9
Healthcare | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Net sales	483.0	369.7
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	483.0	369.7
Notebooks/Mobile Devices
Disaggregation of Revenue [Line Items]
Net sales	$ 858.7	$ 735.5
Revenue net, by Product and Service, Percentage	23.80%	22.60%
Netcomm Products
Disaggregation of Revenue [Line Items]
Net sales	$ 482.9	$ 458.6
Revenue net, by Product and Service, Percentage	13.40%	14.10%
Desktops
Disaggregation of Revenue [Line Items]
Net sales	$ 303.5	$ 263.2
Revenue net, by Product and Service, Percentage	8.40%	8.10%
Video
Disaggregation of Revenue [Line Items]
Net sales	$ 259.2	$ 235.7
Revenue net, by Product and Service, Percentage	7.20%	7.20%
Enterprise and Data Storage (Including Drives)
Disaggregation of Revenue [Line Items]
Net sales	$ 256.0	$ 234.8
Revenue net, by Product and Service, Percentage	7.10%	7.20%
Other Hardware
Disaggregation of Revenue [Line Items]
Net sales	$ 751.8	$ 711.6
Revenue net, by Product and Service, Percentage	20.80%	21.90%
Hardware
Disaggregation of Revenue [Line Items]
Net sales	$ 2,912.1	$ 2,639.4
Revenue net, by Product and Service, Percentage	80.70%	81.10%
Hardware | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 1,251.6	$ 1,146.5
Hardware | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	272.8	240.8
Hardware | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,008.2	958.6
Hardware | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	379.5	293.5
Software
Disaggregation of Revenue [Line Items]
Net sales	$ 505.1	$ 447.9
Revenue net, by Product and Service, Percentage	14.00%	13.80%
Software | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 219.6	$ 202.0
Software | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	43.1	39.9
Software | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	183.1	162.8
Software | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	59.3	43.2
Services
Disaggregation of Revenue [Line Items]
Net sales	$ 161.2	$ 141.3
Revenue net, by Product and Service, Percentage	4.50%	4.30%
Services | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 78.1	$ 75.8
Services | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	6.3	6.2
Services | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	34.6	28.0
Services | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	42.2	31.3
Other
Disaggregation of Revenue [Line Items]
Net sales	$ 28.0	$ 27.4
Revenue net, by Product and Service, Percentage	0.80%	0.80%
Other | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 16.5	$ 16.3
Other | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	5.4	5.1
Other | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	4.1	4.3
Other | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	2.0	1.7
United States
Disaggregation of Revenue [Line Items]
Net sales	3,131.2	2,891.3
United States | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,564.4	1,439.5
United States | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	327.6	292.0
United States | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,230.0	1,153.7
United States | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	9.2	6.1
Rest of World
Disaggregation of Revenue [Line Items]
Net sales	475.2	364.7
Rest of World | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1.4	1.1
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	$ 473.8	$ 363.6