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Segment Information - Disaggregation of Revenue (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Disaggregation of Revenue [Line Items]        
Net sales $ 4,629.9 $ 4,186.1 $ 8,587.8 $ 7,792.5
Percentage of Total Net Sales 100.00% 100.00% 100.00% 100.00%
Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales $ 1,883.9 $ 1,733.8 $ 3,620.1 $ 3,299.6
Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 377.4 329.5 733.0 657.1
Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 1,840.1 1,635.4 3,170.8 2,865.4
Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 528.5 487.4 1,063.9 970.4
Transferred at a point in time where CDW is principal        
Disaggregation of Revenue [Line Items]        
Net sales 4,254.2 3,871.9 7,889.3 7,184.5
Transferred at a point in time where CDW is principal | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 1,708.2 1,583.4 3,290.0 3,009.7
Transferred at a point in time where CDW is principal | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 355.5 309.6 690.7 619.0
Transferred at a point in time where CDW is principal | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 1,725.5 1,537.3 2,968.7 2,681.2
Transferred at a point in time where CDW is principal | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 465.0 441.6 939.9 874.6
Transferred at a point in time where CDW is agent        
Disaggregation of Revenue [Line Items]        
Net sales 217.2 183.6 400.3 349.0
Transferred at a point in time where CDW is agent | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 118.7 101.5 222.7 194.2
Transferred at a point in time where CDW is agent | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 20.0 17.1 39.8 33.6
Transferred at a point in time where CDW is agent | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 65.0 55.3 111.3 96.4
Transferred at a point in time where CDW is agent | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 13.5 9.7 26.5 24.8
Transferred over time where CDW is principal        
Disaggregation of Revenue [Line Items]        
Net sales 158.5 130.6 298.2 259.0
Transferred over time where CDW is principal | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 57.0 48.9 107.4 95.7
Transferred over time where CDW is principal | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 1.9 2.8 2.5 4.5
Transferred over time where CDW is principal | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 49.6 42.8 90.8 87.8
Transferred over time where CDW is principal | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 50.0 36.1 97.5 71.0
Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 1,883.9 1,733.8 3,620.1 3,299.6
Corporate | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 1,883.9 1,733.8 3,620.1 3,299.6
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 377.4 329.5 733.0 657.1
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 377.4 329.5 733.0 657.1
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 578.4 493.5 1,066.8 912.0
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 578.4 493.5 1,066.8 912.0
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 773.6 712.1 1,174.0 1,109.3
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 773.6 712.1 1,174.0 1,109.3
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 488.1 429.8 930.0 844.1
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 488.1 429.8 930.0 844.1
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 528.5 487.4 1,063.9 970.4
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 528.5 487.4 1,063.9 970.4
Notebooks/Mobile Devices        
Disaggregation of Revenue [Line Items]        
Net sales $ 1,232.9 $ 1,078.0 $ 2,210.1 $ 1,943.9
Percentage of Total Net Sales 26.60% 25.80% 25.70% 24.90%
Netcomm Products        
Disaggregation of Revenue [Line Items]        
Net sales $ 563.4 $ 498.7 $ 1,075.8 $ 986.0
Percentage of Total Net Sales 12.20% 11.90% 12.50% 12.70%
Desktops        
Disaggregation of Revenue [Line Items]        
Net sales $ 431.3 $ 353.6 $ 734.9 $ 664.3
Percentage of Total Net Sales 9.30% 8.40% 8.60% 8.50%
Enterprise and Data Storage (Including Drives)        
Disaggregation of Revenue [Line Items]        
Net sales $ 273.0 $ 268.8 $ 563.7 $ 518.8
Percentage of Total Net Sales 5.90% 6.40% 6.60% 6.70%
Video        
Disaggregation of Revenue [Line Items]        
Net sales $ 348.5 $ 323.5 $ 620.1 $ 584.2
Percentage of Total Net Sales 7.50% 7.70% 7.20% 7.50%
Other Hardware        
Disaggregation of Revenue [Line Items]        
Net sales $ 876.1 $ 864.6 $ 1,658.9 $ 1,601.5
Percentage of Total Net Sales 18.90% 20.80% 19.30% 20.70%
Hardware        
Disaggregation of Revenue [Line Items]        
Net sales $ 3,725.2 $ 3,387.2 $ 6,863.5 $ 6,298.7
Percentage of Total Net Sales 80.40% 81.00% 79.90% 81.00%
Hardware | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales $ 1,496.8 $ 1,382.3 $ 2,880.9 $ 2,633.0
Hardware | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 316.4 273.7 611.6 546.8
Hardware | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 1,523.0 1,338.7 2,586.1 2,345.7
Hardware | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 389.0 392.5 784.9 773.2
Software        
Disaggregation of Revenue [Line Items]        
Net sales $ 650.6 $ 603.3 $ 1,244.4 $ 1,107.0
Percentage of Total Net Sales 14.10% 14.30% 14.50% 14.10%
Software | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales $ 270.0 $ 254.2 $ 517.1 $ 473.3
Software | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 48.6 42.9 96.8 85.6
Software | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 262.2 255.4 483.3 439.7
Software | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 69.8 50.8 147.2 108.4
Services        
Disaggregation of Revenue [Line Items]        
Net sales $ 223.4 $ 166.3 $ 420.0 $ 329.3
Percentage of Total Net Sales 4.80% 4.00% 4.90% 4.20%
Services | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales $ 99.4 $ 79.9 $ 187.5 $ 159.3
Services | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 7.0 7.7 14.1 14.0
Services | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 50.1 37.0 92.3 71.6
Services | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 66.9 41.7 126.1 84.4
Other        
Disaggregation of Revenue [Line Items]        
Net sales $ 30.7 $ 29.3 $ 59.9 $ 57.5
Percentage of Total Net Sales 0.70% 0.70% 0.70% 0.70%
Other | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales $ 17.7 $ 17.4 $ 34.6 $ 34.0
Other | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 5.4 5.2 10.5 10.7
Other | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 4.8 4.3 9.1 8.4
Other | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 2.8 2.4 5.7 4.4
United States        
Disaggregation of Revenue [Line Items]        
Net sales 4,108.1 3,704.3 7,535.6 6,835.5
United States | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 1,881.7 1,731.4 3,614.6 3,295.8
United States | Operating Segments | Small Business        
Disaggregation of Revenue [Line Items]        
Net sales 377.4 329.5 733.0 657.1
United States | Operating Segments | Public        
Disaggregation of Revenue [Line Items]        
Net sales 1,840.1 1,635.4 3,170.8 2,865.4
United States | Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Net sales 8.9 8.0 17.2 17.2
Rest of World        
Disaggregation of Revenue [Line Items]        
Net sales 521.8 481.8 1,052.2 957.0
Rest of World | Operating Segments | Corporate        
Disaggregation of Revenue [Line Items]        
Net sales 2.2 2.4 5.5 3.8
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 $ 519.6 $ 479.4 $ 1,046.7 $ 953.2