Segment Information - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 4,956.2	$ 4,756.4	$ 4,365.7	$ 4,389.2	$ 4,536.9	$ 4,907.7	$ 4,629.9	$ 3,957.9	$ 18,467.5	$ 18,032.4	$ 16,240.5
Percentage of Total Net Sales									100.00%	100.00%	100.00%
Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	366.5	337.0	302.1	391.5	391.1	386.2	377.4	355.6
Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 575.7	$ 465.6	$ 484.0	$ 561.4	$ 587.3	$ 507.1	$ 528.5	$ 535.4
Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 6,846.0	$ 7,499.0	$ 6,842.5
Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,397.1	1,510.3	1,359.6
Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									8,137.7	6,864.8	6,154.7
Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									2,086.7	2,158.3	1,883.7
Transferred at a point in time where CDW is principal
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									16,754.9	16,552.6	14,984.0
Transferred at a point in time where CDW is principal | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									6,140.7	6,818.7	6,256.5
Transferred at a point in time where CDW is principal | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,301.3	1,423.1	1,281.3
Transferred at a point in time where CDW is principal | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									7,477.4	6,410.2	5,758.6
Transferred at a point in time where CDW is principal | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,835.5	1,900.6	1,687.6
Transferred at a point in time where CDW is agent
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									896.0	834.2	719.8
Transferred at a point in time where CDW is agent | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									457.4	446.1	389.1
Transferred at a point in time where CDW is agent | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									84.5	80.0	69.4
Transferred at a point in time where CDW is agent | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									292.5	248.5	211.5
Transferred at a point in time where CDW is agent | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									61.6	59.6	49.8
Transferred over time where CDW is principal
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									816.6	645.6	536.7
Transferred over time where CDW is principal | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									247.9	234.2	196.9
Transferred over time where CDW is principal | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									11.3	7.2	8.9
Transferred over time where CDW is principal | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									367.8	206.1	184.6
Transferred over time where CDW is principal | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									189.6	198.1	146.3
Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									6,846.0	7,499.0	6,842.5
Corporate | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									6,846.0	7,499.0	6,842.5
Corporate | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Corporate | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Corporate | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,397.1	1,510.3	1,359.6
Small Business | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Small Business | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,397.1	1,510.3	1,359.6
Small Business | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Small Business | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Government
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									2,978.5	2,519.3	2,097.3
Government | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Government | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Government | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									2,978.5	2,519.3	2,097.3
Government | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Education
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									3,458.1	2,411.6	2,327.4
Education | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Education | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Education | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									3,458.1	2,411.6	2,327.4
Education | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Healthcare
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,701.1	1,933.9	1,730.0
Healthcare | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Healthcare | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Healthcare | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,701.1	1,933.9	1,730.0
Healthcare | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									2,086.7	2,158.3	1,883.7
Other | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Other | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Other | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Other | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									2,086.7	2,158.3	1,883.7
Hardware
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 14,833.4	$ 14,482.2	$ 13,131.7
Percentage of Total Net Sales									80.20%	80.40%	80.90%
Hardware | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 5,289.2	$ 5,963.7	$ 5,464.9
Hardware | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,156.1	1,264.7	1,135.2
Hardware | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									6,844.0	5,624.9	5,039.3
Hardware | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,544.1	1,628.9	1,492.3
Software
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 2,581.0	$ 2,585.0	$ 2,299.1
Percentage of Total Net Sales									14.00%	14.30%	14.20%
Software | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 1,088.3	$ 1,069.2	$ 973.3
Software | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									189.3	196.0	175.2
Software | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									982.8	1,019.6	937.0
Software | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									320.6	300.2	213.6
Services
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 913.9	$ 840.9	$ 695.9
Percentage of Total Net Sales									4.90%	4.70%	4.30%
Services | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 400.8	$ 395.8	$ 336.9
Services | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									31.5	28.5	28.1
Services | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									269.8	199.0	161.8
Services | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									211.8	217.6	169.1
Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 139.2	$ 124.3	$ 113.8
Percentage of Total Net Sales									0.90%	0.60%	0.60%
Other | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 67.7	$ 70.3	$ 67.4
Other | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									20.2	21.1	21.1
Other | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									41.1	21.3	16.6
Other | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									10.2	11.6	8.7
United States
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									16,379.2	15,893.3	14,379.6
United States | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									6,823.6	7,485.7	6,834.4
United States | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									1,397.1	1,510.3	1,359.6
United States | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									8,137.7	6,864.8	6,154.7
United States | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									20.8	32.5	30.9
Rest of World
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									2,088.3	2,139.1	1,860.9
Rest of World | Operating Segments | Corporate
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									22.4	13.3	8.1
Rest of World | Operating Segments | Small Business
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Rest of World | Operating Segments | Public
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									0.0	0.0	0.0
Rest of World | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax									$ 2,065.9	$ 2,125.8	$ 1,852.8