Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Net sales	$ 3,957.9	$ 3,606.4
Revenue net, by Product and Service, Percentage	100.00%	100.00%
Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 1,736.2	$ 1,565.8
Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	355.6	327.6
Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,330.7	1,230.0
Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	535.4	483.0
Transferred at a point in time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales	3,635.2	3,340.1
Transferred at a point in time where CDW is principal | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,581.8	1,426.3
Transferred at a point in time where CDW is principal | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	335.2	309.4
Transferred at a point in time where CDW is principal | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,243.3	1,171.4
Transferred at a point in time where CDW is principal | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	474.9	433.0
Transferred at a point in time where CDW is agent
Disaggregation of Revenue [Line Items]
Net sales	183.0	165.4
Transferred at a point in time where CDW is agent | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	104.0	92.7
Transferred at a point in time where CDW is agent | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	19.8	16.5
Transferred at a point in time where CDW is agent | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	46.2	41.1
Transferred at a point in time where CDW is agent | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	13.0	15.1
Transferred over time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales	139.7	100.9
Transferred over time where CDW is principal | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	50.4	46.8
Transferred over time where CDW is principal | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.6	1.7
Transferred over time where CDW is principal | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	41.2	17.5
Transferred over time where CDW is principal | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	47.5	34.9
Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,736.2	1,565.8
Corporate | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,736.2	1,565.8
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	355.6	327.6
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	355.6	327.6
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	488.4	418.5
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	488.4	418.5
Government | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Education
Disaggregation of Revenue [Line Items]
Net sales	400.4	397.2
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	400.4	397.2
Education | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Healthcare
Disaggregation of Revenue [Line Items]
Net sales	441.9	414.3
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	441.9	414.3
Healthcare | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Net sales	535.4	483.0
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	535.4	483.0
Notebooks/Mobile Devices
Disaggregation of Revenue [Line Items]
Net sales	$ 977.1	$ 865.9
Revenue net, by Product and Service, Percentage	24.70%	24.00%
Netcomm Products
Disaggregation of Revenue [Line Items]
Net sales	$ 512.3	$ 487.3
Revenue net, by Product and Service, Percentage	12.90%	13.50%
Desktops
Disaggregation of Revenue [Line Items]
Net sales	$ 303.7	$ 310.8
Revenue net, by Product and Service, Percentage	7.70%	8.60%
Video
Disaggregation of Revenue [Line Items]
Net sales	$ 271.8	$ 260.8
Revenue net, by Product and Service, Percentage	6.90%	7.20%
Enterprise and Data Storage (Including Drives)
Disaggregation of Revenue [Line Items]
Net sales	$ 290.6	$ 250.1
Revenue net, by Product and Service, Percentage	7.30%	7.00%
Other Hardware
Disaggregation of Revenue [Line Items]
Net sales	$ 782.4	$ 736.6
Revenue net, by Product and Service, Percentage	19.80%	20.40%
Hardware
Disaggregation of Revenue [Line Items]
Net sales	$ 3,137.9	$ 2,911.5
Revenue net, by Product and Service, Percentage	79.30%	80.70%
Hardware | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 1,383.9	$ 1,250.3
Hardware | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	295.0	273.1
Hardware | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,063.1	1,007.0
Hardware | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	395.9	381.1
Software
Disaggregation of Revenue [Line Items]
Net sales	$ 593.9	$ 504.0
Revenue net, by Product and Service, Percentage	15.00%	14.00%
Software | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 247.4	$ 219.6
Software | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	48.3	42.8
Software | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	221.0	184.3
Software | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	77.2	57.3
Services
Disaggregation of Revenue [Line Items]
Net sales	$ 196.6	$ 162.9
Revenue net, by Product and Service, Percentage	5.00%	4.50%
Services | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 88.1	$ 79.3
Services | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	7.0	6.3
Services | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	42.3	34.6
Services | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	59.2	42.7
Other
Disaggregation of Revenue [Line Items]
Net sales	$ 29.5	$ 28.0
Revenue net, by Product and Service, Percentage	0.70%	0.80%
Other | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 16.8	$ 16.6
Other | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	5.3	5.4
Other | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	4.3	4.1
Other | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	3.1	1.9
United States
Disaggregation of Revenue [Line Items]
Net sales	3,427.6	3,131.2
United States | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,732.9	1,564.4
United States | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	355.6	327.6
United States | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,330.7	1,230.0
United States | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	8.4	9.2
Rest of World
Disaggregation of Revenue [Line Items]
Net sales	530.3	475.2
Rest of World | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	3.3	1.4
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	$ 527.0	$ 473.8