Financing receivables that are current.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=66008989&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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Recorded Investment
-URI http://asc.fasb.org/extlink&oid=65880944

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66008989&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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Recorded Investment
-URI http://asc.fasb.org/extlink&oid=65880944

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953401-111524

+ 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=65884687&loc=d3e4531-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765

+ 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_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:

- 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_RealEstateChurchRelatedMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cass_RealEstateChurchRelatedConstructionLoanMember
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:

Loans by Type (Schedule of the Aging of Loans by Loan Categories) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Participating Mortgage Loans [Line Items]
Current	$ 684,233	$ 655,920
Nonaccrual	384	3,135
Loans	684,617	659,055
Current [Member]
Participating Mortgage Loans [Line Items]
Current	$ 684,233	$ 655,920
30 to 59 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
60 to 89 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
90 Days and Over [Member]
Participating Mortgage Loans [Line Items]
Past due
Commercial and Industrial [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	$ 209,663	$ 193,430
Commercial and Industrial [Member] \| Current [Member]
Participating Mortgage Loans [Line Items]
Current	$ 209,663	$ 193,430
Commercial and Industrial [Member] \| 30 to 59 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Commercial and Industrial [Member] \| 60 to 89 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Commercial and Industrial [Member] \| 90 Days and Over [Member]
Participating Mortgage Loans [Line Items]
Past due
Real Estate Commercial Mortgage [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual	$ 290	$ 3,032
Loans	106,656	108,836
Real Estate Commercial Mortgage [Member] \| Current [Member]
Participating Mortgage Loans [Line Items]
Current	$ 106,366	$ 105,804
Real Estate Commercial Mortgage [Member] \| 30 to 59 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Real Estate Commercial Mortgage [Member] \| 60 to 89 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Real Estate Commercial Mortgage [Member] \| 90 Days and Over [Member]
Participating Mortgage Loans [Line Items]
Past due
Real Estate Commercial Construction [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	$ 9,857	$ 1,182
Real Estate Commercial Construction [Member] \| Current [Member]
Participating Mortgage Loans [Line Items]
Current	$ 9,857	$ 1,182
Real Estate Commercial Construction [Member] \| 30 to 59 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Real Estate Commercial Construction [Member] \| 60 to 89 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Real Estate Commercial Construction [Member] \| 90 Days and Over [Member]
Participating Mortgage Loans [Line Items]
Past due
Real Estate Church Related Mortgage [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual	$ 94	$ 103
Loans	306,155	306,728
Real Estate Church Related Mortgage [Member] \| Current [Member]
Participating Mortgage Loans [Line Items]
Current	$ 306,061	$ 306,625
Real Estate Church Related Mortgage [Member] \| 30 to 59 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Real Estate Church Related Mortgage [Member] \| 60 to 89 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Real Estate Church Related Mortgage [Member] \| 90 Days and Over [Member]
Participating Mortgage Loans [Line Items]
Past due
Real Estate Church Related Construction [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	$ 33,259	$ 28,957
Real Estate Church Related Construction [Member] \| Current [Member]
Participating Mortgage Loans [Line Items]
Current	$ 33,259	$ 28,957
Real Estate Church Related Construction [Member] \| 30 to 59 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Real Estate Church Related Construction [Member] \| 60 to 89 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Real Estate Church Related Construction [Member] \| 90 Days and Over [Member]
Participating Mortgage Loans [Line Items]
Past due
Industrial Revenue Bonds [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	$ 19,018	$ 19,831
Industrial Revenue Bonds [Member] \| Current [Member]
Participating Mortgage Loans [Line Items]
Current	$ 19,018	$ 19,831
Industrial Revenue Bonds [Member] \| 30 to 59 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Industrial Revenue Bonds [Member] \| 60 to 89 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Industrial Revenue Bonds [Member] \| 90 Days and Over [Member]
Participating Mortgage Loans [Line Items]
Past due
Other [Member]
Participating Mortgage Loans [Line Items]
Nonaccrual
Loans	$ 9	$ 91
Other [Member] \| Current [Member]
Participating Mortgage Loans [Line Items]
Current	$ 9	$ 91
Other [Member] \| 30 to 59 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Other [Member] \| 60 to 89 Days Past Due [Member]
Participating Mortgage Loans [Line Items]
Past due
Other [Member] \| 90 Days and Over [Member]
Participating Mortgage Loans [Line Items]
Past due