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Segment Information - Disaggregation of Revenue (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disaggregation of Revenue [Line Items]                      
Net sales $ 4,074.8 $ 4,373.2 $ 4,186.1 $ 3,606.4 $ 3,752.1 $ 3,933.2 $ 3,891.7 $ 3,256.0 $ 16,240.5 $ 14,832.9 $ 13,672.7
Percentage of Total Net Sales                 100.00% 100.00% 100.00%
Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales 362.5 340.0 329.5 327.6 308.1 305.4 315.0 292.0      
Other                      
Disaggregation of Revenue [Line Items]                      
Net sales $ 461.7 $ 451.6 $ 487.4 $ 483.0 $ 418.6 $ 381.1 $ 363.8 $ 369.7      
Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 6,842.5 $ 6,172.8 $ 5,734.9
Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,359.6 1,220.5 1,118.1
Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 6,154.7 5,906.5 5,477.4
Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,883.7 1,533.1 1,342.3
Transferred at a point in time where CDW is principal                      
Disaggregation of Revenue [Line Items]                      
Net sales                 14,984.0 13,728.5 12,725.3
Transferred at a point in time where CDW is principal | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 6,256.5 5,640.9 5,279.8
Transferred at a point in time where CDW is principal | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,281.3 1,152.5 1,061.3
Transferred at a point in time where CDW is principal | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 5,758.6 5,559.4 5,182.4
Transferred at a point in time where CDW is principal | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,687.6 1,375.7 1,201.8
Transferred at a point in time where CDW is agent                      
Disaggregation of Revenue [Line Items]                      
Net sales                 719.8 615.6 500.0
Transferred at a point in time where CDW is agent | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 389.1 344.2 281.4
Transferred at a point in time where CDW is agent | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 69.4 59.4 46.8
Transferred at a point in time where CDW is agent | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 211.5 184.1 149.3
Transferred at a point in time where CDW is agent | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 49.8 27.9 22.5
Transferred over time where CDW is principal                      
Disaggregation of Revenue [Line Items]                      
Net sales                 536.7 488.8 447.4
Transferred over time where CDW is principal | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 196.9 187.7 173.7
Transferred over time where CDW is principal | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 8.9 8.6 10.0
Transferred over time where CDW is principal | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 184.6 163.0 145.7
Transferred over time where CDW is principal | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 146.3 129.5 118.0
Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 6,842.5 6,172.8 5,734.9
Corporate | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 6,842.5 6,172.8 5,734.9
Corporate | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Corporate | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Corporate | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,359.6 1,220.5 1,118.1
Small Business | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Small Business | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,359.6 1,220.5 1,118.1
Small Business | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Small Business | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Government                      
Disaggregation of Revenue [Line Items]                      
Net sales                 2,097.3 2,109.8 1,813.6
Government | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 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                 2,097.3 2,109.8 1,813.6
Government | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Education                      
Disaggregation of Revenue [Line Items]                      
Net sales                 2,327.4 2,184.5 1,994.4
Education | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Education | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Education | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 2,327.4 2,184.5 1,994.4
Education | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Healthcare                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,730.0 1,612.2 1,669.4
Healthcare | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Healthcare | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Healthcare | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,730.0 1,612.2 1,669.4
Healthcare | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,883.7 1,533.1 1,342.3
Other | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Other | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Other | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 0.0 0.0 0.0
Other | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,883.7 1,533.1 1,342.3
Notebooks/Mobile Devices                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 4,053.6 $ 3,519.8 $ 2,942.9
Percentage of Total Net Sales                 25.00% 23.70% 21.50%
Netcomm Products                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 2,119.8 $ 2,040.3 $ 1,957.0
Percentage of Total Net Sales                 13.10% 13.80% 14.30%
Desktops                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 1,318.2 $ 1,207.0 $ 1,087.7
Percentage of Total Net Sales                 8.10% 8.10% 8.00%
Video                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 1,185.6 $ 1,078.4 $ 963.0
Percentage of Total Net Sales                 7.30% 7.30% 7.00%
Enterprise and Data Storage (Including Drives)                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 1,099.2 $ 1,087.3 $ 1,073.9
Percentage of Total Net Sales                 6.80% 7.30% 7.90%
Other Hardware                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 3,306.0 $ 3,027.6 $ 2,891.5
Percentage of Total Net Sales                 20.30% 20.40% 21.10%
Hardware                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 13,082.4 $ 11,960.4 $ 10,916.0
Percentage of Total Net Sales                 80.60% 80.60% 79.80%
Hardware | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 5,455.6 $ 4,871.6 $ 4,495.6
Hardware | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,135.8 1,012.5 908.3
Hardware | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 4,998.9 4,846.5 4,481.2
Hardware | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,492.1 1,229.8 1,030.9
Software                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 2,347.0 $ 2,156.9 $ 2,072.3
Percentage of Total Net Sales                 14.40% 14.50% 15.20%
Software | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 982.3 $ 918.5 $ 876.3
Software | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 174.5 163.1 165.0
Software | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 976.4 908.3 855.4
Software | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 213.8 167.0 175.6
Services                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 697.3 $ 602.7 $ 564.2
Percentage of Total Net Sales                 4.30% 4.10% 4.10%
Services | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 337.3 $ 316.2 $ 291.5
Services | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 28.2 24.5 24.1
Services | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 162.8 133.5 119.6
Services | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 169.0 128.5 129.0
Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 113.8 $ 112.9 $ 120.2
Percentage of Total Net Sales                 0.70% 0.80% 0.90%
Other | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 67.3 $ 66.5 $ 71.5
Other | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 21.1 20.4 20.7
Other | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 16.6 18.2 21.2
Other | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 8.8 7.8 6.8
United States                      
Disaggregation of Revenue [Line Items]                      
Net sales                 14,379.6 13,319.9 12,218.3
United States | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 6,834.4 6,167.4 5,594.6
United States | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,359.6 1,220.5 1,118.1
United States | Operating Segments | Public                      
Disaggregation of Revenue [Line Items]                      
Net sales                 6,154.7 5,906.5 5,477.4
United States | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 30.9 25.5 28.2
Rest of World                      
Disaggregation of Revenue [Line Items]                      
Net sales                 1,860.9 1,513.0 1,454.4
Rest of World | Operating Segments | Corporate                      
Disaggregation of Revenue [Line Items]                      
Net sales                 8.1 5.4 140.3
Rest of World | Operating Segments | Small Business                      
Disaggregation of Revenue [Line Items]                      
Net sales                 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
Rest of World | Operating Segments | Other                      
Disaggregation of Revenue [Line Items]                      
Net sales                 $ 1,852.8 $ 1,507.6 $ 1,314.1