Summary of Significant Accounting Policies (Policies)	12 Months Ended
Dec. 31, 2013
Summary of Significant Accounting Policies [Abstract]
Summary of Operations	*Summary of Operations* Cass Information Systems, Inc. (the "Company") provides payment and information services, which include processing and payment of transportation, energy, telecommunications and environmental invoices. These services include the acquisition and management of data, information delivery and financial exchange. The consolidated balance sheet captions, "Accounts and drafts payable" and "Payments in excess of funding," represent the Company's resulting financial position related to the payment services that are performed for customers. The Company also provides a full range of banking services to individual, corporate and institutional customers through Cass Commercial Bank (the "Bank"), its wholly owned bank subsidiary.
Basis of Presentation	*Basis of Presentation* The accounting and reporting policies of the Company and its subsidiaries conform to U.S. generally accepted accounting principles. The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries after elimination of intercompany transactions. Certain amounts in the 2012 and 2011 consolidated financial statements have been reclassified to conform to the 2013 presentation. Such reclassifications have no effect on previously reported net income or shareholders' equity. The Company issued a 10% stock dividend on December 14, 2012. The share and per share information have been restated for this dividend unless indicated otherwise for all periods presented in the accompanying consolidated financial statements.
Use of Estimates	*Use of Estimates* In preparing the consolidated financial statements, Company management is required to make estimates and assumptions which significantly affect the reported amounts in the consolidated financial statements.
Cash and Cash Equivalents	*Cash and Cash Equivalents* For purposes of the consolidated statements of cash flows, the Company considers cash and due from banks, interest-bearing deposits in other financial institutions, federal funds sold and other short-term investments as segregated in the accompanying consolidated balance sheets to be cash equivalents.
Investment in Debt Securities	*Investment in Debt Securities* The Company classifies its debt marketable securities as available-for-sale. Securities classified as available-for-sale are carried at fair value. Unrealized gains and losses, net of the related tax effect, are excluded from earnings and reported in accumulated other comprehensive income, a component of shareholders' equity. A decline in the fair value of any available-for-sale security below cost that is deemed other than temporary results in a charge to earnings and the establishment of a new cost basis for the security. To determine whether impairment is other than temporary, the Company considers whether it is more likely than not that the Company will not be required to sell prior to recovery of the amortized cost basis. Evidence considered in this assessment includes the reasons for impairment, the severity and duration of the impairment, changes in value subsequent to year-end and forecasted performance of the investee. Premiums and discounts are amortized or accreted to interest income over the estimated lives of the securities using the level-yield method. Interest income is recognized when earned. Gains and losses are calculated using the specific identification method.
Allowance for Loan Losses	*Allowance for Loan Losses* The allowance for loan losses is increased by provisions charged to expense and is available to absorb charge-offs, net of recoveries. Management utilizes a systematic, documented approach in determining the appropriate level of the allowance for loan losses. Management's approach, which provides for general and specific allocations, is based on current economic conditions, past losses, collection experience, risk characteristics of the portfolio, assessments of collateral values by obtaining independent appraisals for significant properties, and such other factors which, in management's judgment, deserve current recognition in estimating loan losses. Management believes the allowance for loan losses is adequate to absorb probable losses in the loan portfolio. While management uses all available information to recognize losses on loans, future additions to the allowance may be necessary based on changes in economic conditions. Additionally, various regulatory agencies, as an integral part of their examination process, periodically review the Company's allowance for loan losses. Such agencies may require the Company to increase the allowance for loan losses based on their judgments and interpretations about information available to them at the time of their examination.
Premises and Equipment	*Premises and Equipment* Premises and equipment are stated at cost less accumulated depreciation and amortization. Depreciation is computed over the estimated useful lives of the assets, or the respective lease terms for leasehold improvements, using straight-line and accelerated methods. Estimated useful lives do not exceed 40 years for buildings, the lesser of 10 years or the life of the lease for leasehold improvements and range from 3 to 7 years for software, equipment, furniture and fixtures. Maintenance and repairs are charged to expense as incurred.
Intangible Assets	*Intangible Assets* Cost in excess of fair value of net assets acquired has resulted from business acquisitions. Goodwill and intangible assets with indefinite useful lives are not amortized, but instead are tested for impairment at least annually. Intangible assets with definite useful lives are amortized on a straight-line basis over their respective estimated useful lives. Periodically, the Company reviews intangible assets for events or changes in circumstances that may indicate that the carrying amount of the assets may not be recoverable. Based on those reviews, adjustments of recorded amounts have not been required.
Non-marketable Equity Investments	*Non-marketable Equity Investments* The Company accounts for non-marketable equity investments, in which it holds less than a 20% ownership, under the cost method. Under the cost method of accounting, investments are carried at cost and are adjusted only for other than temporary declines in fair value, distributions of earnings and additional investments. The Company periodically evaluates whether any declines in fair value of its investments are other than temporary. In performing this evaluation, the Company considers various factors including any decline in market price, where available, the investee's financial condition, results of operations, operating trends and other financial ratios. Non-marketable equity investments are included in other assets on the consolidated balance sheets.
Foreclosed Assets	*Foreclosed Assets* Real estate acquired as a result of foreclosure is initially recorded at fair value less estimated selling costs. Fair value is generally determined through the receipt of appraisals. Any write down to fair value at the time the property is acquired is recorded as a charge-off to the allowance for loan losses. Any decline in the fair value of the property subsequent to acquisition is recorded as a charge to non-interest expense.
Treasury Stock	*Treasury Stock* Purchases of the Company's common stock are recorded at cost. Upon reissuance, treasury stock is reduced based upon the average cost basis of shares held.
Comprehensive Income	*Comprehensive Income* Comprehensive income consists of net income, changes in net unrealized gains (losses) on available-for-sale securities and pension liability adjustments and is presented in the accompanying consolidated statements of shareholders' equity and consolidated statements of comprehensive income.
Loans	*Loans* Interest on loans is recognized based upon the principal amounts outstanding. It is the Company's policy to discontinue the accrual of interest when there is reasonable doubt as to the collectability of principal or interest. Subsequent payments received on such loans are applied to principal if there is any doubt as to the collectability of such principal; otherwise, these receipts are recorded as interest income. The accrual of interest on a loan is resumed when the loan is current as to payment of both principal and interest and/or the borrower demonstrates the ability to pay and remain current. Loan origination and commitment fees on originated loans, net of certain direct loan origination costs, are deferred and amortized to interest income using the level-yield method over the estimated lives of the related loans.
Impairment of Loans	*Impairment of Loans* A loan is considered impaired when it is probable that a creditor will be unable to collect all amounts due, both principal and interest, according to the contractual terms of the loan agreement. When measuring impairment, the expected future cash flows of an impaired loan are discounted at the loan's effective interest rate. Alternatively, impairment could be measured by reference to an observable market price, if one exists, or the fair value of the collateral for a collateral-dependent loan. Regardless of the historical measurement method used, the Company measures impairment based on the fair value of the collateral when the Company determines foreclosure is probable. Additionally, impairment of a restructured loan is measured by discounting the total expected future cash flows at the loan's effective rate of interest as stated in the original loan agreement. The Company uses its nonaccrual methods as discussed above for recognizing interest on impaired loans.
Information Services Revenue	*Information Services Revenue* A majority of the Company's revenues are attributable to fees for providing services. These services include transportation invoice rating, payment processing, auditing, and the generation of accounting, transportation and environmental information. The Company also processes, pays and generates management information from electric, gas, telecommunications and other invoices. The specific payment and information processing services provided to each customer are developed individually to meet each customer's specific requirements. The Company enters into service agreements with customers typically for fixed fees per transaction that are invoiced monthly. Revenues are recognized in the period services are rendered and earned under the service agreements, as long as collection is reasonably assured.
Income Taxes	*Income Taxes* Deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates in effect for the year in which those temporary differences are expected to be recovered or settled. Deferred tax assets are reduced if necessary, by a deferred tax asset valuation allowance. In the event that management determines it will not be able to realize all or part of net deferred tax assets in the future, the Company adjusts the recorded value of deferred tax assets, which would result in a direct charge to income tax expense in the period that such determination is made. Likewise, the Company will reverse the valuation allowance when realization of the deferred tax asset is expected. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date.
Earnings Per Share	*Earnings Per Share* Basic earnings per share is computed by dividing net income by the weighted average number of common shares outstanding. Diluted earnings per share is computed by dividing net income by the sum of the weighted average number of common shares outstanding and the weighted average number of potential common shares outstanding.
Share-Based Compensation	*Stock-Based Compensation* The Company follows Financial Accounting Standards Board ("FASB") Accounting Standards Codification ("ASC"), "Accounting for Stock Options and Other Stock-based Compensation" ("ASC 718"), which requires that all stock-based compensation be recognized as an expense in the financial statements and that such cost be measured at the fair value of the award. FASB ASC 718 also requires that excess tax benefits related to stock option exercises and restricted stock awards be reflected as financing cash inflows instead of operating cash inflows.
Pension Plans	*Pension Plans* The amounts recognized in the consolidated financial statements related to pension are determined from actuarial valuations. Inherent in these valuations are assumptions including expected return on plan assets, discount rates at which the liabilities could be settled at December 31, 2013, rate of increase in future compensation levels and mortality rates. These assumptions are updated annually and are disclosed in Note 10. The Company follows FASB ASC 715, "Compensation - Retirement Benefits" ("ASC 715") which requires companies to recognize the overfunded or underfunded status of a defined benefit postretirement plan as an asset or liability in its consolidated balance sheet and to recognize changes in that funded status in the year in which the changes occur through comprehensive income. The funded status is measured as the difference between the fair value of the plan assets and the projected benefit obligation as of the date of its fiscal year-end. There have been no significant changes in the Company's long-term rate of return assumptions for the past three fiscal years ended December 31, 2013 and management believes they are not reasonably likely to change in the future.
Fair Value Measurements	*Fair Value Measurements* The Company follows the provisions of FASB ASC 820, "Fair Value Measurements and Disclosures", which defines fair value, establishes a framework for measuring fair value in generally accepted accounting principles, and outlines disclosures about fair value measurements. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. A three-level hierarchy for valuation techniques is used to measure financial assets and financial liabilities at fair value. This hierarchy is based on whether the valuation inputs are observable or unobservable. Financial instrument valuations are considered Level 1 when they are based on quoted prices in active markets for identical assets or liabilities. Level 2 financial instrument valuations use quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data. Financial instrument valuations are considered Level 3 when they are determined using pricing models, discounted cash flow methodologies or similar techniques and at least one significant model assumption or input is unobservable, and when determination of the fair value requires significant management judgment or estimation. The Company records securities available for sale at their fair values on a recurring basis using Level 2 valuations. Additionally, the Company records impaired loans and other real estate owned at their fair value on a nonrecurring basis. The nonrecurring fair value adjustments typically involve application of lower-of-cost-or-market accounting or impairment write-downs of individual assets.
Impact of New and Not Yet Adopted Accounting Pronouncements	Impact of New and Not Yet Adopted Accounting Pronouncements The new accounting pronouncements are not applicable to the Company and/or do not materially impact the Company.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2013

Summary of Significant Accounting Policies [Abstract]

Summary of Operations Cass Information Systems, Inc. (the "Company") provides payment and information services, which include processing and payment of transportation, energy, telecommunications and environmental invoices. These services include the acquisition and management of data, information delivery and financial exchange. The consolidated balance sheet captions, "Accounts and drafts payable" and "Payments in excess of funding," represent the Company's resulting financial position related to the payment services that are performed for customers. The Company also provides a full range of banking services to individual, corporate and institutional customers through Cass Commercial Bank (the "Bank"), its wholly owned bank subsidiary.

Basis of Presentation

Basis of Presentation The accounting and reporting policies of the Company and its subsidiaries conform to U.S. generally accepted accounting principles. The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries after elimination of intercompany transactions. Certain amounts in the 2012 and 2011 consolidated financial statements have been reclassified to conform to the 2013 presentation. Such reclassifications have no effect on previously reported net income or shareholders' equity. The Company issued a 10% stock dividend on December 14, 2012. The share and per share information have been restated for this dividend unless indicated otherwise for all periods presented in the accompanying consolidated financial statements.

Use of Estimates

Use of Estimates In preparing the consolidated financial statements, Company management is required to make estimates and assumptions which significantly affect the reported amounts in the consolidated financial statements.

Cash and Cash Equivalents

Cash and Cash Equivalents For purposes of the consolidated statements of cash flows, the Company considers cash and due from banks, interest-bearing deposits in other financial institutions, federal funds sold and other short-term investments as segregated in the accompanying consolidated balance sheets to be cash equivalents.

Investment in Debt Securities

Investment in Debt Securities The Company classifies its debt marketable securities as available-for-sale. Securities classified as available-for-sale are carried at fair value. Unrealized gains and losses, net of the related tax effect, are excluded from earnings and reported in accumulated other comprehensive income, a component of shareholders' equity. A decline in the fair value of any available-for-sale security below cost that is deemed other than temporary results in a charge to earnings and the establishment of a new cost basis for the security. To determine whether impairment is other than temporary, the Company considers whether it is more likely than not that the Company will not be required to sell prior to recovery of the amortized cost basis. Evidence considered in this assessment includes the reasons for impairment, the severity and duration of the impairment, changes in value subsequent to year-end and forecasted performance of the investee. Premiums and discounts are amortized or accreted to interest income over the estimated lives of the securities using the level-yield method. Interest income is recognized when earned. Gains and losses are calculated using the specific identification method.

Allowance for Loan Losses

Allowance for Loan Losses The allowance for loan losses is increased by provisions charged to expense and is available to absorb charge-offs, net of recoveries. Management utilizes a systematic, documented approach in determining the appropriate level of the allowance for loan losses. Management's approach, which provides for general and specific allocations, is based on current economic conditions, past losses, collection experience, risk characteristics of the portfolio, assessments of collateral values by obtaining independent appraisals for significant properties, and such other factors which, in management's judgment, deserve current recognition in estimating loan losses.

Management believes the allowance for loan losses is adequate to absorb probable losses in the loan portfolio. While management uses all available information to recognize losses on loans, future additions to the allowance may be necessary based on changes in economic conditions. Additionally, various regulatory agencies, as an integral part of their examination process, periodically review the Company's allowance for loan losses. Such agencies may require the Company to increase the allowance for loan losses based on their judgments and interpretations about information available to them at the time of their examination.

Premises and Equipment

Premises and Equipment Premises and equipment are stated at cost less accumulated depreciation and amortization. Depreciation is computed over the estimated useful lives of the assets, or the respective lease terms for leasehold improvements, using straight-line and accelerated methods. Estimated useful lives do not exceed 40 years for buildings, the lesser of 10 years or the life of the lease for leasehold improvements and range from 3 to 7 years for software, equipment, furniture and fixtures. Maintenance and repairs are charged to expense as incurred.

Intangible Assets

Intangible Assets Cost in excess of fair value of net assets acquired has resulted from business acquisitions. Goodwill and intangible assets with indefinite useful lives are not amortized, but instead are tested for impairment at least annually. Intangible assets with definite useful lives are amortized on a straight-line basis over their respective estimated useful lives.

Periodically, the Company reviews intangible assets for events or changes in circumstances that may indicate that the carrying amount of the assets may not be recoverable. Based on those reviews, adjustments of recorded amounts have not been required.

Non-marketable Equity Investments

Non-marketable Equity Investments The Company accounts for non-marketable equity investments, in which it holds less than a 20% ownership, under the cost method. Under the cost method of accounting, investments are carried at cost and are adjusted only for other than temporary declines in fair value, distributions of earnings and additional investments. The Company periodically evaluates whether any declines in fair value of its investments are other than temporary. In performing this evaluation, the Company considers various factors including any decline in market price, where available, the investee's financial condition, results of operations, operating trends and other financial ratios. Non-marketable equity investments are included in other assets on the consolidated balance sheets.

Foreclosed Assets

Foreclosed Assets Real estate acquired as a result of foreclosure is initially recorded at fair value less estimated selling costs. Fair value is generally determined through the receipt of appraisals. Any write down to fair value at the time the property is acquired is recorded as a charge-off to the allowance for loan losses. Any decline in the fair value of the property subsequent to acquisition is recorded as a charge to non-interest expense.

Treasury Stock

Treasury Stock Purchases of the Company's common stock are recorded at cost. Upon reissuance, treasury stock is reduced based upon the average cost basis of shares held.

Comprehensive Income

Comprehensive Income Comprehensive income consists of net income, changes in net unrealized gains (losses) on available-for-sale securities and pension liability adjustments and is presented in the accompanying consolidated statements of shareholders' equity and consolidated statements of comprehensive income.

Loans

Loans Interest on loans is recognized based upon the principal amounts outstanding. It is the Company's policy to discontinue the accrual of interest when there is reasonable doubt as to the collectability of principal or interest. Subsequent payments received on such loans are applied to principal if there is any doubt as to the collectability of such principal; otherwise, these receipts are recorded as interest income. The accrual of interest on a loan is resumed when the loan is current as to payment of both principal and interest and/or the borrower demonstrates the ability to pay and remain current. Loan origination and commitment fees on originated loans, net of certain direct loan origination costs, are deferred and amortized to interest income using the level-yield method over the estimated lives of the related loans.

Impairment of Loans

Impairment of Loans A loan is considered impaired when it is probable that a creditor will be unable to collect all amounts due, both principal and interest, according to the contractual terms of the loan agreement. When measuring impairment, the expected future cash flows of an impaired loan are discounted at the loan's effective interest rate. Alternatively, impairment could be measured by reference to an observable market price, if one exists, or the fair value of the collateral for a collateral-dependent loan. Regardless of the historical measurement method used, the Company measures impairment based on the fair value of the collateral when the Company determines foreclosure is probable. Additionally, impairment of a restructured loan is measured by discounting the total expected future cash flows at the loan's effective rate of interest as stated in the original loan agreement. The Company uses its nonaccrual methods as discussed above for recognizing interest on impaired loans.

Information Services Revenue

Information Services Revenue A majority of the Company's revenues are attributable to fees for providing services. These services include transportation invoice rating, payment processing, auditing, and the generation of accounting, transportation and environmental information. The Company also processes, pays and generates management information from electric, gas, telecommunications and other invoices. The specific payment and information processing services provided to each customer are developed individually to meet each customer's specific requirements. The Company enters into service agreements with customers typically for fixed fees per transaction that are invoiced monthly. Revenues are recognized in the period services are rendered and earned under the service agreements, as long as collection is reasonably assured.

Income Taxes

Income Taxes Deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates in effect for the year in which those temporary differences are expected to be recovered or settled. Deferred tax assets are reduced if necessary, by a deferred tax asset valuation allowance. In the event that management determines it will not be able to realize all or part of net deferred tax assets in the future, the Company adjusts the recorded value of deferred tax assets, which would result in a direct charge to income tax expense in the period that such determination is made. Likewise, the Company will reverse the valuation allowance when realization of the deferred tax asset is expected. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date.

Earnings Per Share

Earnings Per Share Basic earnings per share is computed by dividing net income by the weighted average number of common shares outstanding. Diluted earnings per share is computed by dividing net income by the sum of the weighted average number of common shares outstanding and the weighted average number of potential common shares outstanding.

Share-Based Compensation

Stock-Based Compensation The Company follows Financial Accounting Standards Board ("FASB") Accounting Standards Codification ("ASC"), "Accounting for Stock Options and Other Stock-based Compensation" ("ASC 718"), which requires that all stock-based compensation be recognized as an expense in the financial statements and that such cost be measured at the fair value of the award. FASB ASC 718 also requires that excess tax benefits related to stock option exercises and restricted stock awards be reflected as financing cash inflows instead of operating cash inflows.

Pension Plans

Pension Plans The amounts recognized in the consolidated financial statements related to pension are determined from actuarial valuations. Inherent in these valuations are assumptions including expected return on plan assets, discount rates at which the liabilities could be settled at December 31, 2013, rate of increase in future compensation levels and mortality rates. These assumptions are updated annually and are disclosed in Note 10. The Company follows FASB ASC 715, "Compensation - Retirement Benefits" ("ASC 715") which requires companies to recognize the overfunded or underfunded status of a defined benefit postretirement plan as an asset or liability in its consolidated balance sheet and to recognize changes in that funded status in the year in which the changes occur through comprehensive income. The funded status is measured as the difference between the fair value of the plan assets and the projected benefit obligation as of the date of its fiscal year-end. There have been no significant changes in the Company's long-term rate of return assumptions for the past three fiscal years ended December 31, 2013 and management believes they are not reasonably likely to change in the future.

Fair Value Measurements

Fair Value Measurements The Company follows the provisions of FASB ASC 820, "Fair Value Measurements and Disclosures", which defines fair value, establishes a framework for measuring fair value in generally accepted accounting principles, and outlines disclosures about fair value measurements. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. A three-level hierarchy for valuation techniques is used to measure financial assets and financial liabilities at fair value. This hierarchy is based on whether the valuation inputs are observable or unobservable. Financial instrument valuations are considered Level 1 when they are based on quoted prices in active markets for identical assets or liabilities. Level 2 financial instrument valuations use quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data. Financial instrument valuations are considered Level 3 when they are determined using pricing models, discounted cash flow methodologies or similar techniques and at least one significant model assumption or input is unobservable, and when determination of the fair value requires significant management judgment or estimation. The Company records securities available for sale at their fair values on a recurring basis using Level 2 valuations. Additionally, the Company records impaired loans and other real estate owned at their fair value on a nonrecurring basis. The nonrecurring fair value adjustments typically involve application of lower-of-cost-or-market accounting or impairment write-downs of individual assets.

Impact of New and Not Yet Adopted Accounting Pronouncements

Impact of New and Not Yet Adopted Accounting Pronouncements

The new accounting pronouncements are not applicable to the Company and/or do not materially impact the Company.