Long-Term Debt - Debt Guarantors, Covenants and Restrictions (Details) £ in Millions, $ in Millions	Dec. 31, 2017 USD ($)	Dec. 31, 2017 GBP (£)
Senior secured term loan facility
Debt Instrument [Line Items]
Amount of restricted payment capacity under the loan	$ 1,153
Net leverage ratio	2.62	2.62
Senior secured term loan facility \| Maximum
Debt Instrument [Line Items]
Net leverage ratio	3.25	3.25
CDW UK term loan
Debt Instrument [Line Items]
Amount of restricted payment capacity under the loan	$ 98	£ 73

Amount of restricted net assets of consolidated and unconsolidated subsidiaries as of the end of the most recently completed fiscal year.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e)(3)(ii))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Name:	us-gaap_AmountOfRestrictedNetAssetsForConsolidatedAndUnconsolidatedSubsidiaries
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant