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Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Net sales	$ 4,536.9	$ 4,907.7	$ 4,629.9	$ 3,957.9	$ 4,074.8	$ 4,373.2	$ 4,186.1	$ 3,606.4	$ 18,032.4	$ 16,240.5	$ 14,832.9
Percentage of Total Net Sales									100.00%	100.00%	100.00%
Small Business
Disaggregation of Revenue [Line Items]
Net sales	391.1	386.2	377.4	355.6	362.5	340.0	329.5	327.6
Other
Disaggregation of Revenue [Line Items]
Net sales	$ 587.3	$ 507.1	$ 528.5	$ 535.4	$ 461.7	$ 451.6	$ 487.4	$ 483.0
Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales									$ 7,499.0	$ 6,842.5	$ 6,172.8
Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales									1,510.3	1,359.6	1,220.5
Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales									6,864.8	6,154.7	5,906.5
Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales									2,158.3	1,883.7	1,533.1
Transferred at a point in time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales									16,552.6	14,984.0	13,728.5
Transferred at a point in time where CDW is principal | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales									6,818.7	6,256.5	5,640.9
Transferred at a point in time where CDW is principal | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales									1,423.1	1,281.3	1,152.5
Transferred at a point in time where CDW is principal | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales									6,410.2	5,758.6	5,559.4
Transferred at a point in time where CDW is principal | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales									1,900.6	1,687.6	1,375.7
Transferred at a point in time where CDW is agent
Disaggregation of Revenue [Line Items]
Net sales									834.2	719.8	615.6
Transferred at a point in time where CDW is agent | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales									446.1	389.1	344.2
Transferred at a point in time where CDW is agent | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales									80.0	69.4	59.4
Transferred at a point in time where CDW is agent | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales									248.5	211.5	184.1
Transferred at a point in time where CDW is agent | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales									59.6	49.8	27.9
Transferred over time where CDW is principal
Disaggregation of Revenue [Line Items]
Net sales									645.6	536.7	488.8
Transferred over time where CDW is principal | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales									234.2	196.9	187.7
Transferred over time where CDW is principal | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales									7.2	8.9	8.6
Transferred over time where CDW is principal | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales									206.1	184.6	163.0
Transferred over time where CDW is principal | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales									198.1	146.3	129.5
Corporate
Disaggregation of Revenue [Line Items]
Net sales									7,499.0	6,842.5	6,172.8
Corporate | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales									7,499.0	6,842.5	6,172.8
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									1,510.3	1,359.6	1,220.5
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									1,510.3	1,359.6	1,220.5
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									2,519.3	2,097.3	2,109.8
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	0.0
Government | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales									2,519.3	2,097.3	2,109.8
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									2,411.6	2,327.4	2,184.5
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									2,411.6	2,327.4	2,184.5
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									1,933.9	1,730.0	1,612.2
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									1,933.9	1,730.0	1,612.2
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									2,158.3	1,883.7	1,533.1
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									2,158.3	1,883.7	1,533.1
Hardware
Disaggregation of Revenue [Line Items]
Net sales									$ 14,363.2	$ 13,099.1	$ 11,971.8
Percentage of Total Net Sales									79.80%	80.60%	80.60%
Hardware | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales									$ 5,955.5	$ 5,462.1	$ 4,879.7
Hardware | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales									1,263.2	1,134.6	1,015.9
Hardware | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales									5,584.8	5,010.4	4,846.4
Hardware | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales									1,559.7	1,492.0	1,229.8
Software
Disaggregation of Revenue [Line Items]
Net sales									$ 2,637.2	$ 2,331.9	$ 2,145.4
Percentage of Total Net Sales									14.60%	14.40%	14.50%
Software | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales									$ 1,079.4	$ 976.2	$ 910.4
Software | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales									197.5	175.8	159.7
Software | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales									1,059.4	965.9	908.3
Software | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales									300.9	214.0	167.0
Services
Disaggregation of Revenue [Line Items]
Net sales									$ 907.6	$ 695.8	$ 602.8
Percentage of Total Net Sales									5.00%	4.30%	4.10%
Services | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales									$ 393.8	$ 336.9	$ 316.2
Services | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales									28.4	28.1	24.5
Services | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales									199.3	161.8	133.6
Services | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales									286.1	169.0	128.5
Other
Disaggregation of Revenue [Line Items]
Net sales									$ 124.4	$ 113.7	$ 112.9
Percentage of Total Net Sales									0.60%	0.70%	0.80%
Other | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales									$ 70.3	$ 67.3	$ 66.5
Other | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales									21.2	21.1	20.4
Other | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales									21.3	16.6	18.2
Other | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales									11.6	8.7	7.8
United States
Disaggregation of Revenue [Line Items]
Net sales									15,893.3	14,379.6	13,319.9
United States | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales									7,485.7	6,834.4	6,167.4
United States | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Net sales									1,510.3	1,359.6	1,220.5
United States | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Net sales									6,864.8	6,154.7	5,906.5
United States | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Net sales									32.5	30.9	25.5
Rest of World
Disaggregation of Revenue [Line Items]
Net sales									2,139.1	1,860.9	1,513.0
Rest of World | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Net sales									13.3	8.1	5.4
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									$ 2,125.8	$ 1,852.8	$ 1,507.6

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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