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 | 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 | 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 |
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 | 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 |
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 | 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 | 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 |
Transferred over time where CDW is principal | Operating Segments | Corporate | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 57 | 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 | 36.1 | 97.5 | 71 |
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 |
Corporate | Operating Segments | Public | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Corporate | Operating Segments | Other | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Small Business | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 377.4 | 329.5 | 733 | 657.1 |
Small Business | Operating Segments | Corporate | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Small Business | Operating Segments | Small Business | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 377.4 | 329.5 | 733 | 657.1 |
Small Business | Operating Segments | Public | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Small Business | Operating Segments | Other | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Government | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 578.4 | 493.5 | 1,066.8 | 912 |
Government | Operating Segments | Corporate | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Government | Operating Segments | Small Business | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Government | Operating Segments | Public | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 578.4 | 493.5 | 1,066.8 | 912 |
Government | Operating Segments | Other | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Education | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 773.6 | 712.1 | 1,174 | 1,109.3 |
Education | Operating Segments | Corporate | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Education | Operating Segments | Small Business | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Education | Operating Segments | Public | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 773.6 | 712.1 | 1,174 | 1,109.3 |
Education | Operating Segments | Other | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Healthcare | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 488.1 | 429.8 | 930 | 844.1 |
Healthcare | Operating Segments | Corporate | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Healthcare | Operating Segments | Small Business | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Healthcare | Operating Segments | Public | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 488.1 | 429.8 | 930 | 844.1 |
Healthcare | Operating Segments | Other | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 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 |
Other | Operating Segments | Small Business | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 0 | 0 | 0 | 0 |
Other | Operating Segments | Public | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 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 | $ 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 |
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 | $ 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 |
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 | 1,338.7 | 2,586.1 | 2,345.7 |
Hardware | Operating Segments | Other | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 389 | 392.5 | 784.9 | 773.2 |
Software | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | $ 650.6 | $ 603.3 | $ 1,244.4 | $ 1,107 |
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 | $ 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 | $ 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 | 7.7 | 14.1 | 14 |
Services | Operating Segments | Public | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 50.1 | 37 | 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 |
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 | 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 | 17.2 | 17.2 |
Rest of World | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 521.8 | 481.8 | 1,052.2 | 957 |
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 |
Rest of World | Operating Segments | Public | | | | |
Disaggregation of Revenue [Line Items] | | | | |
Net sales | 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 |