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Stock-Based Compensation - Summary of RSU Activity (Parenthetical) (Details) shares in Thousands	6 Months Ended
	Jun. 30, 2021 shares
RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs vested but not yet delivered	64

X

- Definition

Share based compensation arrangement by share based payment award equity instruments other than options vested but not yet delivered.

+ References

No definition available.

+ Details

Name:	cars_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedButNotYetDelivered
Namespace Prefix:	cars_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

X

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

X

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: