Financing receivables that are current.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953401-111524

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded Investment in financing receivables that are past due at the balance sheet date.

+ References

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentPastDue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

An amount representing an agreement for an unconditional promise by the maker to pay the Company (holder) a definite sum of money within one year from the balance sheet date (or the normal operating cycle, whichever is longer), net of any write-downs taken for collection uncertainty on the part of the holder. Such amount may include accrued interest receivable in accordance with the terms of the debt. The debt also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. This amount does not include amounts related to receivables held-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4531-111522

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_NotesAndLoansReceivableNetCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cass_CommercialAndIndustrialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cass_RealEstateCommercialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cass_RealEstateCommercialConstructionLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cass_RealEstateFaithbasedMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cass_RealEstateFaithbasedConstructionLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cass_IndustrialRevenueBondMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cass_OtherFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables1To29DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables30To59DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables60To89DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans by Type (Schedule of the Aging of Loans by Loan Categories) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	726,239	686,231
Commercial and Industrial [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	270,854	236,394
Real Estate Commercial Mortgage [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	92,972	94,675
Real Estate Commercial Construction [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	22,883	9,359
Real Estate Faith-based Mortgage [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	307,679	316,073
Real Estate Faith-based Construction [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	30,838	25,948
Industrial Revenue Bonds [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	921	3,374
Other Financing Receivable [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	92	408
Current [Member]
Participating Mortgage Loans [Line Items]
Current	726,239	686,231
Current [Member] \| Commercial and Industrial [Member]
Participating Mortgage Loans [Line Items]
Current	270,854	236,394
Current [Member] \| Real Estate Commercial Mortgage [Member]
Participating Mortgage Loans [Line Items]
Current	92,972	94,675
Current [Member] \| Real Estate Commercial Construction [Member]
Participating Mortgage Loans [Line Items]
Current	22,883	9,359
Current [Member] \| Real Estate Faith-based Mortgage [Member]
Participating Mortgage Loans [Line Items]
Current	307,679	316,073
Current [Member] \| Real Estate Faith-based Construction [Member]
Participating Mortgage Loans [Line Items]
Current	30,838	25,948
Current [Member] \| Industrial Revenue Bonds [Member]
Participating Mortgage Loans [Line Items]
Current	921	3,374
Current [Member] \| Other Financing Receivable [Member]
Participating Mortgage Loans [Line Items]
Current	92	408
30 to 59 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
30 to 59 Days Past Due [Member] \| Commercial and Industrial [Member]
Participating Mortgage Loans [Line Items]
Past due
30 to 59 Days Past Due [Member] \| Real Estate Commercial Mortgage [Member]
Participating Mortgage Loans [Line Items]
Past due
30 to 59 Days Past Due [Member] \| Real Estate Commercial Construction [Member]
Participating Mortgage Loans [Line Items]
Past due
30 to 59 Days Past Due [Member] \| Real Estate Faith-based Mortgage [Member]
Participating Mortgage Loans [Line Items]
Past due
30 to 59 Days Past Due [Member] \| Real Estate Faith-based Construction [Member]
Participating Mortgage Loans [Line Items]
Past due
30 to 59 Days Past Due [Member] \| Industrial Revenue Bonds [Member]
Participating Mortgage Loans [Line Items]
Past due
30 to 59 Days Past Due [Member] \| Other Financing Receivable [Member]
Participating Mortgage Loans [Line Items]
Past due
60 to 89 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
60 to 89 Days Past Due [Member] \| Commercial and Industrial [Member]
Participating Mortgage Loans [Line Items]
Past due
60 to 89 Days Past Due [Member] \| Real Estate Commercial Mortgage [Member]
Participating Mortgage Loans [Line Items]
Past due
60 to 89 Days Past Due [Member] \| Real Estate Commercial Construction [Member]
Participating Mortgage Loans [Line Items]
Past due
60 to 89 Days Past Due [Member] \| Real Estate Faith-based Mortgage [Member]
Participating Mortgage Loans [Line Items]
Past due
60 to 89 Days Past Due [Member] \| Real Estate Faith-based Construction [Member]
Participating Mortgage Loans [Line Items]
Past due
60 to 89 Days Past Due [Member] \| Industrial Revenue Bonds [Member]
Participating Mortgage Loans [Line Items]
Past due
60 to 89 Days Past Due [Member] \| Other Financing Receivable [Member]
Participating Mortgage Loans [Line Items]
Past due
90 Days and Over [Member]
Participating Mortgage Loans [Line Items]
Past due
90 Days and Over [Member] \| Commercial and Industrial [Member]
Participating Mortgage Loans [Line Items]
Past due
90 Days and Over [Member] \| Real Estate Commercial Mortgage [Member]
Participating Mortgage Loans [Line Items]
Past due
90 Days and Over [Member] \| Real Estate Commercial Construction [Member]
Participating Mortgage Loans [Line Items]
Past due
90 Days and Over [Member] \| Real Estate Faith-based Mortgage [Member]
Participating Mortgage Loans [Line Items]
Past due
90 Days and Over [Member] \| Real Estate Faith-based Construction [Member]
Participating Mortgage Loans [Line Items]
Past due
90 Days and Over [Member] \| Industrial Revenue Bonds [Member]
Participating Mortgage Loans [Line Items]
Past due
90 Days and Over [Member] \| Other Financing Receivable [Member]
Participating Mortgage Loans [Line Items]
Past due