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Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 4,756.4 | $ 4,907.7 | $ 13,511.3 | $ 13,495.5
Percentage of Total Net Sales | 100.00% | 100.00% | 100.00% | 100.00%
Operating Segments | Corporate |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 1,660.0 | $ 1,913.5 | $ 5,128.5 | $ 5,533.6
Operating Segments | Small Business |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 337.0 | 386.2 | 1,030.6 | 1,119.2
Operating Segments | Public |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 2,293.8 | 2,100.9 | 5,841.2 | 5,271.7
Operating Segments | Other |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 465.6 | 507.1 | 1,511.0 | 1,571.0
Transferred at a point in time where CDW is principal |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 4,276.1 | 4,490.5 | 12,235.7 | 12,379.8
Transferred at a point in time where CDW is principal | Operating Segments | Corporate |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 1,474.2 | 1,743.4 | 4,602.4 | 5,033.4
Transferred at a point in time where CDW is principal | Operating Segments | Small Business |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 313.9 | 364.0 | 961.3 | 1,054.7
Transferred at a point in time where CDW is principal | Operating Segments | Public |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 2,080.3 | 1,939.3 | 5,339.1 | 4,908.0
Transferred at a point in time where CDW is principal | Operating Segments | Other |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 407.7 | 443.8 | 1,332.9 | 1,383.7
Transferred at a point in time where CDW is agent |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 245.4 | 225.7 | 654.7 | 626.0
Transferred at a point in time where CDW is agent | Operating Segments | Corporate |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 121.3 | 111.0 | 341.1 | 333.7
Transferred at a point in time where CDW is agent | Operating Segments | Small Business |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 20.9 | 19.6 | 61.8 | 59.4
Transferred at a point in time where CDW is agent | Operating Segments | Public |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 89.3 | 79.6 | 209.9 | 190.9
Transferred at a point in time where CDW is agent | Operating Segments | Other |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 13.9 | 15.5 | 41.9 | 42.0
Transferred over time where CDW is principal |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 234.9 | 191.5 | 620.9 | 489.7
Transferred over time where CDW is principal | Operating Segments | Corporate |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 64.5 | 59.1 | 185.0 | 166.5
Transferred over time where CDW is principal | Operating Segments | Small Business |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 2.2 | 2.6 | 7.5 | 5.1
Transferred over time where CDW is principal | Operating Segments | Public |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 124.2 | 82.0 | 292.2 | 172.8
Transferred over time where CDW is principal | Operating Segments | Other |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 44.0 | 47.8 | 136.2 | 145.3
Corporate |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 1,660.0 | 1,913.5 | 5,128.5 | 5,533.6
Corporate | Operating Segments | Corporate |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 1,660.0 | 1,913.5 | 5,128.5 | 5,533.6
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 | 337.0 | 386.2 | 1,030.6 | 1,119.2
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 | 337.0 | 386.2 | 1,030.6 | 1,119.2
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 | 847.7 | 793.4 | 2,135.9 | 1,860.2
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 | 847.7 | 793.4 | 2,135.9 | 1,860.2
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 | 1,078.2 | 807.0 | 2,431.2 | 1,981.0
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 | 1,078.2 | 807.0 | 2,431.2 | 1,981.0
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 | 367.9 | 500.5 | 1,274.1 | 1,430.5
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 | 367.9 | 500.5 | 1,274.1 | 1,430.5
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 | 465.6 | 507.1 | 1,511.0 | 1,571.0
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 | 465.6 | 507.1 | 1,511.0 | 1,571.0
Hardware |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 3,792.2 | $ 3,941.2 | $ 10,784.0 | $ 10,822.5
Percentage of Total Net Sales | 79.70% | 80.20% | 79.80% | 80.20%
Hardware | Operating Segments | Corporate |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 1,252.0 | $ 1,533.2 | $ 3,978.4 | $ 4,417.2
Hardware | Operating Segments | Small Business |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 281.7 | 326.7 | 853.7 | 938.3
Hardware | Operating Segments | Public |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 1,917.8 | 1,722.8 | 4,855.9 | 4,323.2
Hardware | Operating Segments | Other |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 340.7 | 358.5 | 1,096.0 | 1,143.8
Notebooks/Mobile Devices |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 1,384.7 | $ 1,253.2 | $ 3,923.1 | $ 3,334.2
Percentage of Total Net Sales | 29.10% | 25.50% | 29.00% | 24.70%
Netcomm Products |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 525.8 | $ 566.7 | $ 1,453.5 | $ 1,639.7
Percentage of Total Net Sales | 11.10% | 11.50% | 10.80% | 12.10%
Desktops |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 245.9 | $ 433.2 | $ 882.5 | $ 1,148.6
Percentage of Total Net Sales | 5.20% | 8.80% | 6.50% | 8.50%
Video |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 315.9 | $ 353.4 | $ 877.3 | $ 973.2
Percentage of Total Net Sales | 6.60% | 7.20% | 6.50% | 7.20%
Enterprise and Data Storage (Including Drives) |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 249.1 | $ 283.8 | $ 690.0 | $ 850.8
Percentage of Total Net Sales | 5.20% | 5.80% | 5.10% | 6.30%
Other Hardware |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 1,070.8 | $ 1,050.9 | $ 2,957.6 | $ 2,876.0
Percentage of Total Net Sales | 22.50% | 21.40% | 21.90% | 21.40%
Software |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 706.1 | $ 689.9 | $ 1,941.8 | $ 1,917.1
Percentage of Total Net Sales | 14.80% | 14.10% | 14.40% | 14.20%
Software | Operating Segments | Corporate |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 284.9 | $ 263.6 | $ 799.2 | $ 777.4
Software | Operating Segments | Small Business |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 43.3 | 47.4 | 139.5 | 144.2
Software | Operating Segments | Public |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 304.7 | 313.2 | 764.3 | 783.1
Software | Operating Segments | Other |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 73.2 | 65.7 | 238.8 | 212.4
Services |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 227.4 | $ 243.8 | $ 689.7 | $ 662.9
Percentage of Total Net Sales | 4.80% | 5.00% | 5.10% | 4.90%
Services | Operating Segments | Corporate |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 107.1 | $ 98.2 | $ 300.0 | $ 285.9
Services | Operating Segments | Small Business |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 7.0 | 6.7 | 22.4 | 20.7
Services | Operating Segments | Public |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 63.8 | 58.7 | 197.9 | 150.0
Services | Operating Segments | Other |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 49.5 | 80.2 | 169.4 | 206.3
Other |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 30.7 | $ 32.8 | $ 95.8 | $ 93.0
Percentage of Total Net Sales | 0.70% | 0.70% | 0.70% | 0.70%
Other | Operating Segments | Corporate |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | $ 16.0 | $ 18.5 | $ 50.9 | $ 53.1
Other | Operating Segments | Small Business |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 5.0 | 5.4 | 15.0 | 16.0
Other | Operating Segments | Public |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 7.5 | 6.2 | 23.1 | 15.4
Other | Operating Segments | Other |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 2.2 | 2.7 | 6.8 | 8.5
United States |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 4,295.3 | 4,405.6 | 11,997.0 | 11,941.2
United States | Operating Segments | Corporate |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 1,658.4 | 1,910.9 | 5,109.1 | 5,525.5
United States | Operating Segments | Small Business |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 337.0 | 386.2 | 1,030.6 | 1,119.2
United States | Operating Segments | Public |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 2,293.8 | 2,100.9 | 5,841.2 | 5,271.7
United States | Operating Segments | Other |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 6.1 | 7.6 | 16.1 | 24.8
Rest of World |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 461.1 | 502.1 | 1,514.3 | 1,554.3
Rest of World | Operating Segments | Corporate |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Net sales | 1.6 | 2.6 | 19.4 | 8.1
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 | $ 459.5 | $ 499.5 | $ 1,494.9 | $ 1,546.2

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