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

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

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

- 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

- 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_RealEstateFaithBasedRelatedMortgageMember
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_IndustrialRevenueBondsMember
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 (Schedule of the Aging of Loans by Loan Categories) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non Accrual
Total Loans	721,587	686,231
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non Accrual
Total Loans	277,091	236,394
Real Estate Commercial Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non Accrual
Total Loans	95,605	94,675
Real Estate Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non Accrual
Total Loans	11,858	9,359
Real Estate Faith based Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non Accrual
Total Loans	316,147	316,073
Real Estate Faith-based Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non Accrual
Total Loans	20,576	25,948
Industrial Revenue Bonds [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non Accrual
Total Loans		3,374
Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non Accrual
Total Loans	310	408
Current [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	721,587	686,231
Current [Member] \| Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	277,091	236,394
Current [Member] \| Real Estate Commercial Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	95,605	94,675
Current [Member] \| Real Estate Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	11,858	9,359
Current [Member] \| Real Estate Faith based Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	316,147	316,073
Current [Member] \| Real Estate Faith-based Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	20,576	25,948
Current [Member] \| Industrial Revenue Bonds [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans		3,374
Current [Member] \| Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans	310	408
30-59 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
30-59 Days [Member] \| Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
30-59 Days [Member] \| Real Estate Commercial Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
30-59 Days [Member] \| Real Estate Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
30-59 Days [Member] \| Real Estate Faith based Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
30-59 Days [Member] \| Real Estate Faith-based Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
30-59 Days [Member] \| Industrial Revenue Bonds [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
30-59 Days [Member] \| Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
60-89 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
60-89 Days [Member] \| Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
60-89 Days [Member] \| Real Estate Commercial Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
60-89 Days [Member] \| Real Estate Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
60-89 Days [Member] \| Real Estate Faith based Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
60-89 Days [Member] \| Real Estate Faith-based Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
60-89 Days [Member] \| Industrial Revenue Bonds [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
60-89 Days [Member] \| Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
90 Days and over [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
90 Days and over [Member] \| Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
90 Days and over [Member] \| Real Estate Commercial Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
90 Days and over [Member] \| Real Estate Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
90 Days and over [Member] \| Real Estate Faith based Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
90 Days and over [Member] \| Real Estate Faith-based Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
90 Days and over [Member] \| Industrial Revenue Bonds [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans
90 Days and over [Member] \| Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans