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Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2021
Percentage holding required to account investments under non-marketable equity investments	20.00%
Buildings | Maximum
Premises, useful life	40 years
Leasehold Improvements | Minimum
Premises, useful life	10 years
Property, Plant and Equipment, Other Types | Minimum
Premises, useful life	3 years
Property, Plant and Equipment, Other Types | Maximum
Premises, useful life	7 years

X

- Definition

The percentage holding required to account investments under non-marketable equity investments.

+ References

No definition available.

+ Details

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Namespace Prefix:	cass_
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Balance Type:	na
Period Type:	instant

X

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: