Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Net sales	$ 4,907.7	$ 4,373.2	$ 13,495.5	$ 12,165.7
Percentage of Total Net Sales	100.00%	100.00%	100.00%	100.00%
Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 1,913.5	$ 1,706.5	$ 5,533.6	$ 5,006.1
Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	386.2	340.0	1,119.2	997.1
Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	2,100.9	1,875.1	5,271.7	4,740.5
Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	507.1	451.6	1,571.0	1,422.0
Transferred at a point in time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales	4,490.5	4,060.0	12,379.8	11,244.5
Transferred at a point in time where CDW is principal | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,743.4	1,571.6	5,033.4	4,581.3
Transferred at a point in time where CDW is principal | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	364.0	321.0	1,054.7	940.0
Transferred at a point in time where CDW is principal | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,939.3	1,764.6	4,908.0	4,445.8
Transferred at a point in time where CDW is principal | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	443.8	402.8	1,383.7	1,277.4
Transferred at a point in time where CDW is agent
Disaggregation of Revenue [Line Items]
Net sales	225.7	179.2	626.0	528.2
Transferred at a point in time where CDW is agent | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	111.0	85.7	333.7	279.9
Transferred at a point in time where CDW is agent | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	19.6	16.9	59.4	50.5
Transferred at a point in time where CDW is agent | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	79.6	64.3	190.9	160.7
Transferred at a point in time where CDW is agent | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	15.5	12.3	42.0	37.1
Transferred over time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales	191.5	134.0	489.7	393.0
Transferred over time where CDW is principal | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	59.1	49.2	166.5	144.9
Transferred over time where CDW is principal | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	2.6	2.1	5.1	6.6
Transferred over time where CDW is principal | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	82.0	46.2	172.8	134.0
Transferred over time where CDW is principal | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	47.8	36.5	145.3	107.5
Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,913.5	1,706.5	5,533.6	5,006.1
Corporate | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,913.5	1,706.5	5,533.6	5,006.1
Corporate | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Corporate | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Corporate | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Small Business
Disaggregation of Revenue [Line Items]
Net sales	386.2	340.0	1,119.2	997.1
Small Business | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Small Business | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	386.2	340.0	1,119.2	997.1
Small Business | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Small Business | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Government
Disaggregation of Revenue [Line Items]
Net sales	793.4	639.3	1,860.2	1,551.3
Government | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Government | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Government | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	793.4	639.3	1,860.2	1,551.3
Government | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Education
Disaggregation of Revenue [Line Items]
Net sales	807.0	793.1	1,981.0	1,902.4
Education | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Education | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Education | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	807.0	793.1	1,981.0	1,902.4
Education | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Healthcare
Disaggregation of Revenue [Line Items]
Net sales	500.5	442.7	1,430.5	1,286.8
Healthcare | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Healthcare | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Healthcare | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	500.5	442.7	1,430.5	1,286.8
Healthcare | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Net sales	507.1	451.6	1,571.0	1,422.0
Other | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Other | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Other | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Other | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	507.1	451.6	1,571.0	1,422.0
Notebooks/Mobile Devices
Disaggregation of Revenue [Line Items]
Net sales	$ 1,325.6	$ 1,124.8	$ 3,537.5	$ 3,069.4
Percentage of Total Net Sales	27.00%	25.70%	26.20%	25.20%
Netcomm Products
Disaggregation of Revenue [Line Items]
Net sales	$ 567.3	$ 582.4	$ 1,641.9	$ 1,568.5
Percentage of Total Net Sales	11.60%	13.30%	12.20%	12.90%
Desktops
Disaggregation of Revenue [Line Items]
Net sales	$ 448.4	$ 349.0	$ 1,183.6	$ 1,013.2
Percentage of Total Net Sales	9.10%	8.00%	8.80%	8.30%
Video
Disaggregation of Revenue [Line Items]
Net sales	$ 353.3	$ 320.5	$ 972.7	$ 904.4
Percentage of Total Net Sales	7.20%	7.30%	7.20%	7.40%
Enterprise and Data Storage (Including Drives)
Disaggregation of Revenue [Line Items]
Net sales	$ 283.6	$ 297.4	$ 849.2	$ 816.2
Percentage of Total Net Sales	5.80%	6.80%	6.30%	6.70%
Other Hardware
Disaggregation of Revenue [Line Items]
Net sales	$ 946.0	$ 883.8	$ 2,604.8	$ 2,484.7
Percentage of Total Net Sales	19.30%	20.30%	19.30%	20.50%
Hardware
Disaggregation of Revenue [Line Items]
Net sales	$ 3,924.2	$ 3,557.9	$ 10,789.7	$ 9,856.4
Percentage of Total Net Sales	80.00%	81.40%	80.00%	81.00%
Hardware | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 1,528.0	$ 1,379.5	$ 4,411.6	$ 4,013.4
Hardware | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	326.6	284.0	937.5	829.4
Hardware | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	1,711.3	1,530.8	4,297.6	3,876.7
Hardware | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	358.3	363.6	1,143.0	1,136.9
Software
Disaggregation of Revenue [Line Items]
Net sales	$ 707.2	$ 608.4	$ 1,951.1	$ 1,715.3
Percentage of Total Net Sales	14.30%	13.90%	14.40%	14.10%
Software | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 269.1	$ 227.5	$ 784.8	$ 699.6
Software | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	47.6	44.3	145.0	131.2
Software | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	324.4	291.0	808.0	730.5
Software | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	66.1	45.6	213.3	154.0
Services
Disaggregation of Revenue [Line Items]
Net sales	$ 243.8	$ 179.0	$ 661.9	$ 508.2
Percentage of Total Net Sales	5.00%	4.10%	4.90%	4.20%
Services | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 97.9	$ 83.1	$ 284.0	$ 242.3
Services | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	6.7	6.6	20.8	20.6
Services | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	59.0	48.8	150.8	120.5
Services | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	80.2	40.5	206.3	124.8
Other
Disaggregation of Revenue [Line Items]
Net sales	$ 32.5	$ 27.9	$ 92.8	$ 85.8
Percentage of Total Net Sales	0.70%	0.60%	0.70%	0.70%
Other | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 18.5	$ 16.4	$ 53.2	$ 50.8
Other | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	5.3	5.1	15.9	15.9
Other | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	6.2	4.5	15.3	12.8
Other | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	2.5	1.9	8.4	6.3
United States
Disaggregation of Revenue [Line Items]
Net sales	4,405.6	3,926.0	11,941.2	10,761.5
United States | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	1,910.9	1,704.1	5,525.5	4,999.9
United States | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	386.2	340.0	1,119.2	997.1
United States | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	2,100.9	1,875.1	5,271.7	4,740.5
United States | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	7.6	6.8	24.8	24.0
Rest of World
Disaggregation of Revenue [Line Items]
Net sales	502.1	447.2	1,554.3	1,404.2
Rest of World | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales	2.6	2.4	8.1	6.2
Rest of World | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Rest of World | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Rest of World | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales	$ 499.5	$ 444.8	$ 1,546.2	$ 1,398.0