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Segment Information - Disaggregation of Revenue (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Disaggregation of Revenue [Line Items]        
Net sales $ 4,186.1 $ 3,891.7 $ 7,792.5 $ 7,147.6
Revenue net, by Product and Service, Percentage 100.00% 100.00% 100.00% 100.00%
Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales $ 1,733.8 $ 1,580.1 $ 3,299.6 $ 3,020.7
Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 329.5 315.0 657.1 607.0
Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 1,635.4 1,632.8 2,865.4 2,786.5
Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 487.4 363.8 970.4 733.4
Transferred at a point in time where CDW is principal        
Disaggregation of Revenue [Line Items]        
Net sales 3,871.9 3,599.5 7,184.5 6,601.2
Transferred at a point in time where CDW is principal | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 1,583.4 1,437.9 3,009.7 2,752.2
Transferred at a point in time where CDW is principal | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 309.6 296.7 619.0 572.4
Transferred at a point in time where CDW is principal | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 1,537.3 1,539.3 2,681.2 2,619.7
Transferred at a point in time where CDW is principal | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 441.6 325.6 874.6 656.9
Transferred at a point in time where CDW is agent        
Disaggregation of Revenue [Line Items]        
Net sales 183.6 167.6 349.0 308.4
Transferred at a point in time where CDW is agent | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 101.5 92.6 194.2 173.8
Transferred at a point in time where CDW is agent | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 17.1 16.0 33.6 29.9
Transferred at a point in time where CDW is agent | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 55.3 52.1 96.4 89.6
Transferred at a point in time where CDW is agent | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 9.7 6.9 24.8 15.1
Transferred over time where CDW is principal        
Disaggregation of Revenue [Line Items]        
Net sales 130.6 124.6 259.0 238.0
Transferred over time where CDW is principal | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 48.9 49.6 95.7 94.7
Transferred over time where CDW is principal | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 2.8 2.3 4.5 4.7
Transferred over time where CDW is principal | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 42.8 41.4 87.8 77.2
Transferred over time where CDW is principal | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 36.1 31.3 71.0 61.4
Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 1,733.8 1,580.1 3,299.6 3,020.7
Corporate | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 1,733.8 1,580.1 3,299.6 3,020.7
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 329.5 315.0 657.1 607.0
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 329.5 315.0 657.1 607.0
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 493.5 523.4 912.0 898.1
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 493.5 523.4 912.0 898.1
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 712.1 704.9 1,109.3 1,098.1
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 712.1 704.9 1,109.3 1,098.1
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 429.8 404.5 844.1 790.3
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 429.8 404.5 844.1 790.3
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 487.4 363.8 970.4 733.4
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 487.4 363.8 970.4 733.4
Notebooks/Mobile Devices        
Disaggregation of Revenue [Line Items]        
Net sales $ 1,068.3 $ 912.8 $ 1,927.0 $ 1,647.1
Revenue net, by Product and Service, Percentage 25.50% 23.50% 24.70% 23.00%
Netcomm Products        
Disaggregation of Revenue [Line Items]        
Net sales $ 498.6 $ 540.3 $ 981.5 $ 999.2
Revenue net, by Product and Service, Percentage 11.90% 13.90% 12.60% 14.00%
Desktops        
Disaggregation of Revenue [Line Items]        
Net sales $ 345.3 $ 308.6 $ 648.8 $ 571.5
Revenue net, by Product and Service, Percentage 8.20% 7.90% 8.30% 8.00%
Video        
Disaggregation of Revenue [Line Items]        
Net sales $ 325.8 $ 289.5 $ 585.0 $ 525.0
Revenue net, by Product and Service, Percentage 7.80% 7.40% 7.50% 7.30%
Enterprise and Data Storage (Including Drives)        
Disaggregation of Revenue [Line Items]        
Net sales $ 264.5 $ 291.3 $ 520.5 $ 525.7
Revenue net, by Product and Service, Percentage 6.30% 7.50% 6.70% 7.40%
Other Hardware        
Disaggregation of Revenue [Line Items]        
Net sales $ 879.7 $ 778.1 $ 1,631.5 $ 1,492.2
Revenue net, by Product and Service, Percentage 21.10% 20.00% 21.00% 20.90%
Hardware        
Disaggregation of Revenue [Line Items]        
Net sales $ 3,382.2 $ 3,120.6 $ 6,294.3 $ 5,760.7
Revenue net, by Product and Service, Percentage 80.80% 80.20% 80.80% 80.60%
Hardware | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales $ 1,380.8 $ 1,238.5 $ 2,632.4 $ 2,385.9
Hardware | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 273.9 261.0 546.7 501.7
Hardware | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 1,337.8 1,329.7 2,346.0 2,288.3
Hardware | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 389.7 291.4 769.2 584.8
Software        
Disaggregation of Revenue [Line Items]        
Net sales $ 605.7 $ 589.7 $ 1,110.8 $ 1,036.6
Revenue net, by Product and Service, Percentage 14.50% 15.20% 14.30% 14.50%
Software | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales $ 254.2 $ 243.5 $ 473.8 $ 444.5
Software | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 42.6 42.5 85.7 82.5
Software | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 255.8 263.9 438.9 426.7
Software | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 53.1 39.8 112.4 82.9
Services        
Disaggregation of Revenue [Line Items]        
Net sales $ 168.8 $ 152.8 $ 330.0 $ 294.2
Revenue net, by Product and Service, Percentage 4.00% 3.90% 4.20% 4.10%
Services | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales $ 81.3 $ 81.3 $ 159.4 $ 157.1
Services | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 7.7 6.2 14.0 12.5
Services | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 37.6 34.7 72.2 62.7
Services | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 42.2 30.6 84.4 61.9
Other        
Disaggregation of Revenue [Line Items]        
Net sales $ 29.4 $ 28.6 $ 57.4 $ 56.1
Revenue net, by Product and Service, Percentage 0.70% 0.70% 0.70% 0.80%
Other | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales $ 17.5 $ 16.8 $ 34.0 $ 33.2
Other | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 5.3 5.3 10.7 10.3
Other | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 4.2 4.5 8.3 8.8
Other | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 2.4 2.0 4.4 3.8
United States        
Disaggregation of Revenue [Line Items]        
Net sales 3,704.3 3,533.4 6,835.5 6,424.8
United States | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 1,731.4 1,578.5 3,295.8 3,018.1
United States | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 329.5 315.0 657.1 607.0
United States | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 1,635.4 1,632.8 2,865.4 2,786.5
United States | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 8.0 7.1 17.2 13.2
Rest of World        
Disaggregation of Revenue [Line Items]        
Net sales 481.8 358.3 957.0 722.8
Rest of World | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 2.4 1.6 3.8 2.6
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 $ 479.4 $ 356.7 $ 953.2 $ 720.2