REVENUE FROM CONTRACTS WITH CUSTOMERS (Contract Assets and Liabilities) (Details) - USD ($) $ in Thousands		12 Months Ended
Jan. 01, 2018	Dec. 31, 2018
Change in Contract with Customer, Liability [Roll Forward]
Beginning balance	$ 45,686	$ 45,686
Cumulative effect of adoption of ASC 606	(1,716)
Revenue recognized in the current period from the amounts in the beginning balance		(43,121)
New deferrals, net of amounts recognized in the current period		51,000
Effects of foreign currency		(390)
Ending balance	$ 43,970	$ 51,459

Change in Contract with Customer, Liability [Roll Forward]

No definition available.

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Period Type:	duration

- Definition

Contract with Customer, Asset, Adoption of New Accounting Principle

+ References

No definition available.

+ Details

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Namespace Prefix:	csgp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Contract with Customer, Asset, Effects of Currency Translation

+ References

No definition available.

+ Details

Name:	csgp_ContractwithCustomerAssetEffectsofCurrencyTranslation
Namespace Prefix:	csgp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Contract with Customer, Liability, New Deferrals, Net of Amounts Recognized in Current Period

+ References

No definition available.

+ Details

Name:	csgp_ContractwithCustomerLiabilityNewDeferralsNetofAmountsRecognizedinCurrentPeriod
Namespace Prefix:	csgp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130531-203044

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130532-203044

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

+ Details

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Balance Type:	credit
Period Type:	duration