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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Percentage holding required to account investments under non-marketable equity investments	20.00%
Right-of-use assets	$ 5,848	$ 7,383
Lease liabilities	$ 6,682	$ 7,808
Building [Member] | Maximum [Member]
Estimated useful lives	40 years
Leasehold Improvements [Member] | Minimum [Member]
Estimated useful lives	10 years
Software, Equipment, Furniture and Fixtures [Member] | Maximum [Member]
Estimated useful lives	7 years
Software, Equipment, Furniture and Fixtures [Member] | Minimum [Member]
Estimated useful lives	3 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_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

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Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of lessee's right to use underlying asset under operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

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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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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:
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X

- Details

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Namespace Prefix:
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Balance Type:
Period Type: