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Employee Benefit Plans - Schedule of Long-term Rate of Return on Assets (Details)	12 Months Ended
	Dec. 31, 2023
Core Fixed Income
Defined Benefit Plan Disclosure [Line Items]
% of Total Portfolio	86.00%
One-Year Nominal Return	4.52%
Annual Standard Deviation	4.30%
Global Equity
Defined Benefit Plan Disclosure [Line Items]
% of Total Portfolio	14.00%
One-Year Nominal Return	8.54%
Annual Standard Deviation	18.12%

X

- Definition

The expected percentage of annual standard deviation on assets.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

X

- Definition

The expected percentage of one-year nominal return on assets.

+ References

No definition available.

+ Details

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Namespace Prefix:	cass_
Data Type:	dtr-types1:percentItemType
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

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X

- Definition

Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480482/715-20-55-17

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480506/715-20-50-1

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X

- Details

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Namespace Prefix:
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X

- Details

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