Equity-Based Compensation Equity Awards Granted by Seller of CDW UK (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	1,021,398
Grants, weighted average exercise price (in dollars per share)	$ 73.85
Payment of incentive compensation plan withholding taxes	$ 33.9	$ 49.6	$ 0.0
CDW UK \| Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	456,613
Grants, weighted average exercise price (in dollars per share)	$ 35.93
Payment of incentive compensation plan withholding taxes	$ 19.0

Amount of cash outflow to satisfy an employee's income tax withholding obligation as part of a net-share settlement of a share-based award.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 25
-Paragraph 19A
-URI http://asc.fasb.org/extlink&oid=107668666&loc=SL79513924-113897

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