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Goodwill and Other Intangible Assets - Schedule of Goodwill by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Beginning balance	$ 2,455.0	$ 2,500.4	$ 2,217.6
Foreign currency translation	24.6	(45.4)	(22.4)
Acquisition			305.2
CDW Advanced Services Allocation		0.0
Ending balance	2,479.6	2,455.0	2,500.4
Corporate
Goodwill [Roll Forward]
Beginning balance	1,074.1	1,045.9	1,045.9
Foreign currency translation	0.0	0.0	0.0
Acquisition			0.0
CDW Advanced Services Allocation		28.2
Ending balance	1,074.1	1,074.1	1,045.9
Accumulated impairment losses		(1,571.4)
Small Business
Goodwill [Roll Forward]
Beginning balance	185.9	185.9	185.9
Foreign currency translation	0.0	0.0	0.0
Acquisition			0.0
CDW Advanced Services Allocation		0.0
Ending balance	185.9	185.9	185.9
Public
Goodwill [Roll Forward]
Beginning balance	929.6	911.3	911.3
Foreign currency translation	0.0	0.0	0.0
Acquisition			0.0
CDW Advanced Services Allocation		18.3
Ending balance	929.6	929.6	911.3
Accumulated impairment losses		(354.1)
Other
Goodwill [Roll Forward]
Beginning balance	265.4	357.3	74.5
Foreign currency translation	24.6	(45.4)	(22.4)
Acquisition			305.2
CDW Advanced Services Allocation		(46.5)
Ending balance	$ 290.0	265.4	$ 357.3
Accumulated impairment losses		$ (28.3)

X

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
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+ References

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-Publisher FASB
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+ References

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