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Employee Benefit Plans - Schedule of Assumptions used to Determine Projected Benefit Obligation of the SERP (Details)	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
SERP
Defined Benefit Plan Disclosure [Line Items]
Weighted average discount rate	5.50%	4.95%	5.15%

X

- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (k)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

+ Details

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Balance Type:	na
Period Type:	instant

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

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

X

- Details

Name:	us-gaap_RetirementPlanTypeAxis=cass_SupplementalExecutiveRetirementPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: