Employee Benefit Plans (Schedule of Long-term Rate of Return on Assets (Details)	12 Months Ended
Dec. 31, 2017
Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent of Total Portfolio	51.00%
One-Year Nominal Return	4.55%
Annual Standard Deviation	4.55%
Large Cap U.S. Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent of Total Portfolio	14.00%
One-Year Nominal Return	7.00%
Annual Standard Deviation	15.85%
Small Cap U.S. Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent of Total Portfolio	5.00%
One-Year Nominal Return	8.10%
Annual Standard Deviation	19.70%
International (Developed) [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent of Total Portfolio	25.00%
One-Year Nominal Return	7.99%
Annual Standard Deviation	17.93%
International (Emerging) [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent of Total Portfolio	5.00%
One-Year Nominal Return	10.56%
Annual Standard Deviation	27.35%

The expected percentage of annual standard deviation on assets.

No definition available.

Name:	cass_AnnualStandardDeviation
Namespace Prefix:	cass_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	cass_OneYearNominalReturn
Namespace Prefix:	cass_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

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

- 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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(i)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_FixedIncomeFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=cass_EquitySecuritiesLargeCapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=cass_EquitySecuritiesSmallCapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=cass_ForeignEquitySecuritiesDevelopedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=cass_ForeignEquitySecuritiesEmergingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: