Acquisitions - Intangible Assets (Details) - R&B $ in Millions	Mar. 11, 2020 USD ($)
Customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible Asset Amount	$ 113.7
Amortization Period	15 years
Discount Rate	10.00%
Attrition Rate	7.50%
Non-compete agreement
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible Asset Amount	$ 0.4
Amortization Period	5 years
Discount Rate	10.00%
Trademark
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible Asset Amount	$ 0.4
Amortization Period	1 year
Discount Rate	10.00%

Acquired Finite-Lived Intangible Assets, Valuation Assumption, Attrition Rate, Percentage

No definition available.

Name:	cnm_AcquiredFiniteLivedIntangibleAssetsValuationAssumptionAttritionRatePercentage
Namespace Prefix:	cnm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Acquired Finite-Lived Intangible Assets, Valuation Assumption, Discount Rate, Percentage

+ References

No definition available.

+ Details

Name:	cnm_AcquiredFiniteLivedIntangibleAssetsValuationAssumptionDiscountRatePercentage
Namespace Prefix:	cnm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

+ Details

Name:	us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

+ Details

Name:	us-gaap_FinitelivedIntangibleAssetsAcquired1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CustomerRelationshipsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_NoncompeteAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TrademarksMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: