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FAIR VALUE (Details)	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Assets:
Total assets measured at fair value	$ 586,831,000	$ 600,637,000
Unobservable inputs assets (level 3) [Roll forward]
Temporary impairment of the auction rates security investments	(730,000)	(730,000)
Money Market Funds
Assets:
Total assets measured at fair value	576,761,000	590,567,000
Auction Rate Securities
Assets:
Total assets measured at fair value	10,070,000	10,070,000
Fair Value, Inputs, Level 1
Assets:
Total assets measured at fair value	576,761,000	590,567,000
Fair Value, Inputs, Level 1 | Money Market Funds
Assets:
Total assets measured at fair value	576,761,000	590,567,000
Fair Value, Inputs, Level 1 | Auction Rate Securities
Assets:
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 2
Assets:
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 2 | Money Market Funds
Assets:
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 2 | Auction Rate Securities
Assets:
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 3
Assets:
Total assets measured at fair value	10,070,000	10,070,000
Fair Value, Inputs, Level 3 | Money Market Funds
Assets:
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 3 | Auction Rate Securities
Assets:
Total assets measured at fair value	10,070,000	10,070,000
Auction Rate Securities
Unobservable inputs assets (level 3) [Roll forward]
Beginning balance	10,070,000	10,070,000
Decrease in unrealized loss included in accumulated other comprehensive loss	0	0
Ending balance	$ 10,070,000	$ 10,070,000
Auction rate securities variable rate debt instruments interest rate reset period	28 days
The minimum contractual maturities on the underlying securities involved in the auction rate securities (in years)	20 years
Par value of company held auction rate securities	$ 11,000,000
Temporary impairment of the auction rates security investments	$ 0
Weighted Average | Measurement Input, Discount Rate
Unobservable inputs assets (level 3) [Roll forward]
Debt securities, measurement input	0.05	0.06

X

- Definition

Auction rate securities are variable rate investments that have the interest rate on the security set to the market rate on a contractual basis, this element defines the contractual basis period for the security market change in the interest rate.

+ References

No definition available.

+ Details

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X

- Definition

This element represents the underlying maturity dates of the securities the auction rate security is based upon.

+ References

No definition available.

+ Details

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Namespace Prefix:	csgp_
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- Definition

Amount, after tax, of accumulated unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e681-108580

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

The value of securities held, for which the interest rate resets through an auction process, that are categorized neither as trading nor held-to-maturity, that failed to be liquidated through auction.

+ References

No definition available.

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- Definition

Value of input used to measure investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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