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Employee Benefit Plans (Schedule of Long-term Rate of Return on Assets (Details)	12 Months Ended
	Dec. 31, 2019
Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent of Total Portfolio	51.00%
One-Year Nominal Return	3.78%
Annual Standard Deviation	3.90%
Large Cap U.S. Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent of Total Portfolio	14.00%
One-Year Nominal Return	6.55%
Annual Standard Deviation	15.30%
Small Cap U.S. Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent of Total Portfolio	5.00%
One-Year Nominal Return	7.58%
Annual Standard Deviation	19.00%
International (Developed) [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent of Total Portfolio	25.00%
One-Year Nominal Return	7.86%
Annual Standard Deviation	17.22%
International (Emerging) [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent of Total Portfolio	5.00%
One-Year Nominal Return	10.24%
Annual Standard Deviation	25.05%

X

- Definition

The expected percentage of annual standard deviation on assets.

+ References

No definition available.

+ Details

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Namespace Prefix:	cass_
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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:	num:percentItemType
Balance Type:	na
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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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- Details

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X

- Details

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- Details

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