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LOANS RECEIVABLE - Loans Modification (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021 USD ($) contract item	Dec. 31, 2020 USD ($) contract item
Financing Receivable, Allowance for Credit Loss [Line Items]
Number of contracts | contract	2	18
Pre-modification Outstanding Recorded Investment	$ 140	$ 2,643
Post-modification Outstanding Recorded Investment	$ 142	$ 2,703
TDR modified maturity term	3 months
Number of payment defaults | item	1	0
Commitment to lend additional fund	$ 0
One- to four-family residential
Financing Receivable, Allowance for Credit Loss [Line Items]
Number of contracts | contract	2	18
Pre-modification Outstanding Recorded Investment	$ 140	$ 2,643
Post-modification Outstanding Recorded Investment	$ 142	$ 2,703

X

- Definition

The amount of commitments to lend additional funds to existing TDR relationships.

+ References

No definition available.

+ Details

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Namespace Prefix:	clst_
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Balance Type:	credit
Period Type:	duration

X

- Definition

Troubled debt restructured loans modified average maturity term.

+ References

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- 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.

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

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- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

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- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

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- Definition

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953682-111524

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