Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Disaggregation of Revenue [Line Items]
Net sales	$ 6,145.8	$ 5,146.4	$ 12,094.9	$ 9,983.9
Percentage of Total Net Sales	100.00%	100.00%	100.00%	100.00%
Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 2,660.7	$ 1,983.3	$ 5,288.3	$ 3,788.9
Small Business
Disaggregation of Revenue [Line Items]
Net sales	500.0	482.9	1,024.0	915.6
Public
Disaggregation of Revenue [Line Items]
Net sales	2,243.0	2,080.7	4,276.0	4,002.4
Other
Disaggregation of Revenue [Line Items]
Net sales	742.1	599.5	1,506.6	1,277.0
Transferred at a point in time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales	5,477.1	4,721.5	10,765.7	9,177.7
Transferred at a point in time where CDW is principal | Corporate
Disaggregation of Revenue [Line Items]
Net sales	2,325.9	1,789.2	4,617.1	3,416.9
Transferred at a point in time where CDW is principal | Small Business
Disaggregation of Revenue [Line Items]
Net sales	456.2	451.6	936.2	853.6
Transferred at a point in time where CDW is principal | Public
Disaggregation of Revenue [Line Items]
Net sales	2,032.4	1,953.5	3,872.7	3,779.4
Transferred at a point in time where CDW is principal | Other
Disaggregation of Revenue [Line Items]
Net sales	662.6	527.2	1,339.7	1,127.8
Transferred at a point in time where CDW is agent
Disaggregation of Revenue [Line Items]
Net sales	328.5	251.4	669.2	472.1
Transferred at a point in time where CDW is agent | Corporate
Disaggregation of Revenue [Line Items]
Net sales	172.3	120.7	352.9	233.0
Transferred at a point in time where CDW is agent | Small Business
Disaggregation of Revenue [Line Items]
Net sales	33.4	25.9	67.7	52.8
Transferred at a point in time where CDW is agent | Public
Disaggregation of Revenue [Line Items]
Net sales	103.5	85.0	199.8	143.1
Transferred at a point in time where CDW is agent | Other
Disaggregation of Revenue [Line Items]
Net sales	19.3	19.8	48.8	43.2
Transferred over time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales	340.2	173.5	660.0	334.1
Transferred over time where CDW is principal | Corporate
Disaggregation of Revenue [Line Items]
Net sales	162.5	73.4	318.3	139.0
Transferred over time where CDW is principal | Small Business
Disaggregation of Revenue [Line Items]
Net sales	10.4	5.4	20.1	9.2
Transferred over time where CDW is principal | Public
Disaggregation of Revenue [Line Items]
Net sales	107.1	42.2	203.5	79.9
Transferred over time where CDW is principal | Other
Disaggregation of Revenue [Line Items]
Net sales	60.2	52.5	118.1	106.0
Corporate
Disaggregation of Revenue [Line Items]
Net sales	2,660.7	1,983.3	5,288.3	3,788.9
Corporate | Corporate
Disaggregation of Revenue [Line Items]
Net sales	2,660.7	1,983.3	5,288.3	3,788.9
Corporate | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Corporate | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Corporate | 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	500.0	482.9	1,024.0	915.6
Small Business | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Small Business | Small Business
Disaggregation of Revenue [Line Items]
Net sales	500.0	482.9	1,024.0	915.6
Small Business | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Small Business | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Government
Disaggregation of Revenue [Line Items]
Net sales	609.5	513.4	1,153.4	1,029.5
Government | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Government | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Government | Public
Disaggregation of Revenue [Line Items]
Net sales	609.5	513.4	1,153.4	1,029.5
Government | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Education
Disaggregation of Revenue [Line Items]
Net sales	1,041.3	1,112.1	1,944.1	2,055.4
Education | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Education | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Education | Public
Disaggregation of Revenue [Line Items]
Net sales	1,041.3	1,112.1	1,944.1	2,055.4
Education | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Healthcare
Disaggregation of Revenue [Line Items]
Net sales	592.2	455.2	1,178.5	917.5
Healthcare | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Healthcare | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Healthcare | Public
Disaggregation of Revenue [Line Items]
Net sales	592.2	455.2	1,178.5	917.5
Healthcare | Other
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Net sales	742.1	599.5	1,506.6	1,277.0
Other | Corporate
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Other | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Other | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Other | Other
Disaggregation of Revenue [Line Items]
Net sales	742.1	599.5	1,506.6	1,277.0
Hardware
Disaggregation of Revenue [Line Items]
Net sales	$ 4,721.9	$ 4,225.6	$ 9,404.0	$ 8,195.2
Percentage of Total Net Sales	76.90%	82.10%	77.90%	82.10%
Hardware | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 1,989.3	$ 1,586.5	$ 3,972.9	$ 3,007.3
Hardware | Small Business
Disaggregation of Revenue [Line Items]
Net sales	419.7	416.4	863.2	784.6
Hardware | Public
Disaggregation of Revenue [Line Items]
Net sales	1,797.1	1,763.9	3,443.4	3,431.8
Hardware | Other
Disaggregation of Revenue [Line Items]
Net sales	515.8	458.8	1,124.5	971.5
Notebooks/Mobile Devices
Disaggregation of Revenue [Line Items]
Net sales	$ 1,763.6	$ 1,710.1	$ 3,477.4	$ 3,302.8
Percentage of Total Net Sales	28.70%	33.20%	28.80%	33.10%
Netcomm Products
Disaggregation of Revenue [Line Items]
Net sales	$ 705.2	$ 483.8	$ 1,191.6	$ 938.4
Percentage of Total Net Sales	11.50%	9.40%	9.90%	9.40%
Desktops
Disaggregation of Revenue [Line Items]
Net sales	$ 350.2	$ 293.7	$ 687.6	$ 558.9
Percentage of Total Net Sales	5.70%	5.70%	5.70%	5.60%
Video
Disaggregation of Revenue [Line Items]
Net sales	$ 526.5	$ 410.5	$ 1,012.5	$ 736.5
Percentage of Total Net Sales	8.60%	8.00%	8.40%	7.40%
Enterprise and Data Storage (Including Drives)
Disaggregation of Revenue [Line Items]
Net sales	$ 323.3	$ 242.5	$ 677.0	$ 502.1
Percentage of Total Net Sales	5.30%	4.70%	5.60%	5.00%
Other Hardware
Disaggregation of Revenue [Line Items]
Net sales	$ 1,053.1	$ 1,085.0	$ 2,357.9	$ 2,156.5
Percentage of Total Net Sales	17.10%	21.10%	19.50%	21.60%
Software
Disaggregation of Revenue [Line Items]
Net sales	$ 920.9	$ 637.8	$ 1,719.4	$ 1,243.3
Percentage of Total Net Sales	15.00%	12.40%	14.20%	12.50%
Software | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 423.0	$ 267.6	$ 822.8	$ 534.6
Software | Small Business
Disaggregation of Revenue [Line Items]
Net sales	55.0	49.4	114.8	98.5
Software | Public
Disaggregation of Revenue [Line Items]
Net sales	288.7	242.5	542.0	433.9
Software | Other
Disaggregation of Revenue [Line Items]
Net sales	154.2	78.3	239.8	176.3
Services
Disaggregation of Revenue [Line Items]
Net sales	$ 469.7	$ 251.7	$ 907.1	$ 485.6
Percentage of Total Net Sales	7.60%	4.90%	7.50%	4.90%
Services | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 228.9	$ 110.9	$ 454.2	$ 213.5
Services | Small Business
Disaggregation of Revenue [Line Items]
Net sales	19.8	11.3	34.9	21.5
Services | Public
Disaggregation of Revenue [Line Items]
Net sales	152.4	70.1	281.5	128.0
Services | Other
Disaggregation of Revenue [Line Items]
Net sales	68.6	59.4	136.5	122.6
Other
Disaggregation of Revenue [Line Items]
Net sales	$ 33.3	$ 31.3	$ 64.4	$ 59.8
Percentage of Total Net Sales	0.50%	0.60%	0.40%	0.50%
Other | Corporate
Disaggregation of Revenue [Line Items]
Net sales	$ 19.5	$ 18.3	$ 38.4	$ 33.5
Other | Small Business
Disaggregation of Revenue [Line Items]
Net sales	5.5	5.8	11.1	11.0
Other | Public
Disaggregation of Revenue [Line Items]
Net sales	4.8	4.2	9.1	8.7
Other | Other
Disaggregation of Revenue [Line Items]
Net sales	3.5	3.0	5.8	6.6
United States
Disaggregation of Revenue [Line Items]
Net sales	5,402.5	4,544.5	10,580.9	8,707.0
United States | Corporate
Disaggregation of Revenue [Line Items]
Net sales	2,655.4	1,976.2	5,272.2	3,779.7
United States | Small Business
Disaggregation of Revenue [Line Items]
Net sales	500.0	482.9	1,024.0	915.6
United States | Public
Disaggregation of Revenue [Line Items]
Net sales	2,243.0	2,080.7	4,276.0	4,002.4
United States | Other
Disaggregation of Revenue [Line Items]
Net sales	4.1	4.7	8.7	9.3
Rest of World
Disaggregation of Revenue [Line Items]
Net sales	743.3	601.9	1,514.0	1,276.9
Rest of World | Corporate
Disaggregation of Revenue [Line Items]
Net sales	5.3	7.1	16.1	9.2
Rest of World | Small Business
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Rest of World | Public
Disaggregation of Revenue [Line Items]
Net sales	0.0	0.0	0.0	0.0
Rest of World | Other
Disaggregation of Revenue [Line Items]
Net sales	$ 738.0	$ 594.8	$ 1,497.9	$ 1,267.7