Goodwill and Other Intangible Assets - Schedule of Goodwill by Segment (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Goodwill [Roll Forward]
Beginning balance	$ 2,595.9	$ 2,553.0
Foreign currency translation	(2.1)	9.9
Ending balance	4,382.9	2,595.9
Corporate
Goodwill [Roll Forward]
Beginning balance	1,123.6	1,090.6
Foreign currency translation	0.0	0.0
Ending balance	2,024.4	1,123.6
Accumulated impairment losses	1,571.0
Small Business
Goodwill [Roll Forward]
Beginning balance	185.9	185.9
Foreign currency translation	0.0	0.0
Ending balance	266.1	185.9
Public
Goodwill [Roll Forward]
Beginning balance	929.6	929.6
Foreign currency translation	0.0	0.0
Ending balance	1,737.7	929.6
Accumulated impairment losses	354.0
Other
Goodwill [Roll Forward]
Beginning balance	356.8	346.9
Foreign currency translation	(2.1)	9.9
Ending balance	354.7	356.8
Accumulated impairment losses	28.0
IGNW, Inc.
Goodwill [Roll Forward]
Acquisition		33.0
IGNW, Inc. \| Corporate
Goodwill [Roll Forward]
Acquisition		33.0
IGNW, Inc. \| Small Business
Goodwill [Roll Forward]
Acquisition		0.0
IGNW, Inc. \| Public
Goodwill [Roll Forward]
Acquisition		0.0
IGNW, Inc. \| Other
Goodwill [Roll Forward]
Acquisition		$ 0.0
Amplified IT LLC
Goodwill [Roll Forward]
Acquisition	133.8
Amplified IT LLC \| Corporate
Goodwill [Roll Forward]
Acquisition	0.0
Amplified IT LLC \| Small Business
Goodwill [Roll Forward]
Acquisition	0.0
Amplified IT LLC \| Public
Goodwill [Roll Forward]
Acquisition	133.8
Amplified IT LLC \| Other
Goodwill [Roll Forward]
Acquisition	0.0
Focal Point Data Risk LLC
Goodwill [Roll Forward]
Acquisition	82.7
Focal Point Data Risk LLC \| Corporate
Goodwill [Roll Forward]
Acquisition	0.0
Focal Point Data Risk LLC \| Small Business
Goodwill [Roll Forward]
Acquisition	0.0
Focal Point Data Risk LLC \| Public
Goodwill [Roll Forward]
Acquisition	82.7
Focal Point Data Risk LLC \| Other
Goodwill [Roll Forward]
Acquisition	0.0
Sirius Computer Solutions, Inc.
Goodwill [Roll Forward]
Acquisition	1,572.4
Sirius Computer Solutions, Inc. \| Corporate
Goodwill [Roll Forward]
Acquisition	900.6
Sirius Computer Solutions, Inc. \| Small Business
Goodwill [Roll Forward]
Acquisition	80.2
Sirius Computer Solutions, Inc. \| Public
Goodwill [Roll Forward]
Acquisition	591.6
Sirius Computer Solutions, Inc. \| Other
Goodwill [Roll Forward]
Acquisition	0.0
Other acquisition adjustments
Goodwill [Roll Forward]
Acquisition	0.2
Other acquisition adjustments \| Corporate
Goodwill [Roll Forward]
Acquisition	0.2
Other acquisition adjustments \| Small Business
Goodwill [Roll Forward]
Acquisition	0.0
Other acquisition adjustments \| Public
Goodwill [Roll Forward]
Acquisition	0.0
Other acquisition adjustments \| Other
Goodwill [Roll Forward]
Acquisition	$ 0.0

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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10)(1))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56426-112766

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

+ Details

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

- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267

+ Details

Name:	us-gaap_GoodwillAcquiredDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267

+ Details

Name:	us-gaap_GoodwillForeignCurrencyTranslationGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of accumulated impairment loss for 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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267

+ Details

Name:	us-gaap_GoodwillImpairedAccumulatedImpairmentLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_GoodwillRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=cdw_SmallBusinessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=cdw_PublicMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_AllOtherSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cdw_IGNWIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cdw_AmplifiedITLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cdw_FocalPointDataRiskLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cdw_SiriusComputerSolutionsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=cdw_OtherAcquisitionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: