Summary Of Significant Accounting Policies (Policies)	12 Months Ended
Summary Of Significant Accounting Policies (Policies)	Sep. 30, 2017
Accounting Policies [Abstract]
Description of Business	Capitol Federal Financial, Inc. (the "Company") provides a full range of retail banking services through its wholly-owned subsidiary, Capitol Federal Savings Bank (the "Bank"), a federal savings bank, which has 37 traditional and 10 in-store banking offices serving primarily the metropolitan areas of Topeka, Wichita, Lawrence, Manhattan, Emporia and Salina, Kansas and portions of the metropolitan area of greater Kansas City. The Bank emphasizes mortgage lending, primarily originating and purchasing one- to four-family loans, and providing personal retail financial services. The Bank is subject to competition from other financial institutions and other companies that provide financial services.
Basis of Presentation	The consolidated financial statements include the accounts of the Company and its wholly owned subsidiary, the Bank. The Bank has a wholly owned subsidiary, Capitol Funds, Inc. Capitol Funds, Inc. has a wholly-owned subsidiary, Capitol Federal Mortgage Reinsurance Company ("CFMRC"). All intercompany accounts and transactions have been eliminated in consolidation. These consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America ("GAAP"), and require management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting periods. Actual results could differ from these estimates and assumptions. The Bank has an expense sharing agreement with the Company that covers the reimbursement of certain expenses that are allocable to the Company. These expenses include compensation, rent for leased office space, and general overhead expenses. The Company, the Bank, Capitol Funds, Inc. and CFMRC have a tax allocation agreement. The Bank is the paying agent to the taxing authorities for the group for all periods presented. Each entity is liable for taxes as if separate tax returns were filed and reimburses the Bank for its pro rata share of the tax liability. If any entity has a tax benefit, the Bank reimburses the entity for its tax benefit.
Cash and Cash Equivalents	Cash and cash equivalents include cash on hand and amounts due from banks. Regulations of the Board of Governors of the Federal Reserve System ("FRB") require federally chartered savings banks to maintain cash reserves against their transaction accounts. Required reserves must be maintained in the form of vault cash, an account at a Federal Reserve Bank, or a pass-through account as defined by the FRB. The amount of interest-earning deposits held at the Federal Reserve Bank of Kansas City ("FRB of Kansas City") as of September 30, 2017 and 2016 was $337.5 million and $264.4 million, respectively. The Bank is in compliance with the FRB requirements. For the years ended September 30, 2017 and 2016, the average daily balance of required reserves at the FRB of Kansas City was $9.1 million and $8.8 million, respectively.
Net Presentation of Cash Flows Related to Borrowings	During the current fiscal year, the Bank entered into certain FHLB advances with contractual maturities of 90 days or less. Cash flows related to these advances are reported on a net basis in the consolidated statements of cash flows.
Securities	Securities include MBS and agency debentures issued primarily by United States Government-Sponsored Enterprises ("GSE"), including Federal National Mortgage Association, Federal Home Loan Mortgage Corporation and the Federal Home Loan Banks, United States Government agencies, including Government National Mortgage Association, and municipal bonds. Securities are classified as HTM, AFS, or trading based on management's intention for holding the securities on the date of purchase. Generally, classifications are made in response to liquidity needs, asset/liability management strategies, and the market interest rate environment at the time of purchase. Securities that management has the intent and ability to hold to maturity are classified as HTM and reported at amortized cost. Such securities are adjusted for the amortization of premiums and discounts which are recognized as adjustments to interest income over the life of the securities using the level-yield method. Securities that management may sell if necessary for liquidity or asset management purposes are classified as AFS and reported at fair value, with unrealized gains and losses reported as a component of AOCI within stockholders' equity, net of deferred income taxes. The amortization of premiums and discounts are recognized as adjustments to interest income over the life of the securities using the level-yield method. Gains or losses on the disposition of AFS securities are recognized using the specific identification method. The Company primarily uses prices obtained from third party pricing services to determine the fair value of securities. See additional discussion of fair value of AFS securities in "Note 13. Fair Value of Financial Instruments." Securities that are purchased and held principally for resale in the near future are classified as trading securities and are reported at fair value, with unrealized gains and losses included in non-interest income in the consolidated statements of income. During the fiscal years ended September 30, 2017 and 2016, neither the Company nor the Bank maintained a trading securities portfolio. Management monitors securities in the investment portfolio for impairment on an ongoing basis and performs a formal review quarterly. The process involves monitoring market events and other items that could impact issuers. The evaluation includes, but is not limited to, such factors as: the nature of the investment, the length of time the security has had a fair value less than the amortized cost basis, the cause(s) and severity of the loss, expectation of an anticipated recovery period, recent events specific to the issuer or industry including the issuer's financial condition and current ability to make future payments in a timely manner, external credit ratings and recent downgrades in such ratings, management's intent to sell and whether it is more likely than not management would be required to sell prior to recovery for debt securities. Management determines whether other-than-temporary losses should be recognized for impaired securities by assessing all known facts and circumstances surrounding the securities. If management intends to sell an impaired security or if it is more likely than not that management will be required to sell an impaired security before recovery of its amortized cost basis, an other-than-temporary impairment has occurred and the difference between amortized cost and fair value will be recognized as a loss in earnings and the security will be written down to fair value.
Loans Receivable	Loans receivable that management has the intent and ability to hold for the foreseeable future are carried at the amount of unpaid principal, net of ACL, undisbursed loan funds, unamortized premiums and discounts, and deferred loan origination fees and costs. Net loan origination fees and costs, and premiums and discounts are amortized as yield adjustments to interest income using the level-yield method. Interest on loans is credited to income as earned and accrued only if deemed collectible. Troubled debt restructurings ("TDRs") - For borrowers experiencing financial difficulties, the Bank may grant a concession to the borrower, resulting in a TDR. Such concessions generally involve extensions of loan maturity dates, the granting of periods during which the payment of only interest and escrow is required, reductions in interest rates, and loans that have been discharged under Chapter 7 bankruptcy proceedings where the borrower has not reaffirmed the debt. The Bank does not forgive principal or interest nor does it commit to lend additional funds, except for situations generally involving the capitalization of delinquent interest and/or escrow not to exceed the original loan balance, to these borrowers. Delinquent loans - A loan is considered delinquent when payment has not been received within 30 days of its contractual due date. The number of days delinquent is determined by the number of scheduled payments that remain unpaid, assuming a period of 30 days between each scheduled payment. Nonaccrual loans - The accrual of income on loans is discontinued when interest or principal payments are 90 days in arrears. We also report certain TDR loans as nonaccrual loans that are required to be reported as such pursuant to regulatory reporting requirements. Loans on which the accrual of income has been discontinued are designated as nonaccrual and outstanding interest previously credited beyond 90 days delinquent is reversed. A nonaccrual loan is returned to accrual status once the contractual payments have been made to bring the loan less than 90 days past due or, in the case of a TDR loan, the borrower has made the required consecutive loan payments. Impaired loans - A loan is considered impaired when, based on current information and events, it is probable that the Bank will be unable to collect all amounts due, including principal and interest, according to the original contractual terms of the loan agreement. Interest income on impaired loans is recognized in the period collected unless the ultimate collection of principal is considered doubtful. Loans reported as impaired loans include loans partially charged-off and TDRs.
Allowance for Credit Losses	The ACL represents management's best estimate of the amount of inherent losses in the loan portfolio as of the balance sheet date. It involves a high degree of complexity and requires management to make difficult and subjective judgments and assumptions about highly uncertain matters. Management's methodology for assessing the appropriateness of the ACL consists of a formula analysis model, along with analyzing and considering several other relevant internal and external data elements. The use of different judgments and assumptions could cause reported results to differ significantly. Management maintains the ACL through provisions for credit losses that are either charged or credited to income. One- to four-family loans, including home equity loans, are individually evaluated for loss when the loan is generally 180 days delinquent and any losses are charged-off. Losses are based on new collateral values obtained through appraisals, less estimated costs to sell. Anticipated private mortgage insurance proceeds are taken into consideration when calculating the loss amount. An updated appraisal is requested, at a minimum, every 12 months thereafter if the loan is 180 days or more delinquent or in foreclosure. If the Bank holds the first and second mortgage, both loans are combined when evaluating whether there is a potential loss on the loan. For commercial real estate loans, losses are charged-off when the collection of such amounts is determined to be unlikely. When a non-real estate secured loan, which includes consumer loans - other, is 120 days delinquent, any identified losses are charged-off. Charge-offs for any loan type may also occur at any time if the Bank has knowledge of the existence of a potential loss. The primary risk characteristics inherent in the one- to four-family and consumer loan portfolios are a decline in economic conditions, elevated levels of unemployment or underemployment, and declines in residential real estate values. Any one or a combination of these events may adversely affect borrowers' ability to repay their loans, resulting in increased delinquencies, non-performing assets, loan losses, and future loan loss provisions. Although the commercial real estate loan portfolio is subject to the same risk of declines in economic conditions, the primary risk characteristics inherent in this portfolio include the ability of the borrower to sustain sufficient cash flows from leases and to control expenses to satisfy their contractual debt payments, and/or the ability to utilize personal and/or business resources to pay their contractual debt payments if the cash flows are not sufficient. Additionally, if the Bank were to repossess the secured collateral of a commercial real estate loan, the pool of potential buyers is typically limited more than that for a residential property. This increases the risk that the Bank could hold the property for an extended period of time and/or potentially be forced to sell at a discounted price, resulting in additional losses. Each quarter end, a formula analysis is prepared which segregates the loan portfolio into categories based on certain risk characteristics. The categories include the following: one- to four-family loans; commercial real estate loans; consumer home equity loans; and other consumer loans. Home equity loans with the same underlying collateral as a one- to four-family loan are combined with the one- to four-family loan in the formula analysis model to calculate a combined loan-to-value ("LTV") ratio. The one- to four-family loan portfolio and related home equity loans are segregated into additional categories based on the following risk characteristics: loan source (originated, correspondent purchased, or bulk purchased), interest payments (fixed-rate and adjustable-rate), LTV ratios, borrower's credit scores, and certain geographic locations. The categories were derived by management based on reviewing the historical performance of the one- to four-family loan portfolio and taking into consideration current economic conditions, such as trends in residential real estate values in certain areas of the U.S. and unemployment rates. Impaired loans are not included in the formula analysis as they are individually evaluated for loss. Historical loss factors are applied to each loan category in the formula analysis model. Each quarter end, management reviews historical losses over a look-back time period and utilizes the historical loss time periods believed to be the most appropriate considering the current economic conditions. The historical loss time period is then adjusted for a loss emergence time period, which represents the estimated time period from the date of a loss event to the date we recognize a charge-off/loss. Qualitative loss factors are utilized in the formula analysis model to reflect risks inherent in each loan category that are not captured by the historical loss factors. The qualitative loss factors for one- to four-family and consumer loan portfolios take into consideration such items as: unemployment rate trends, residential real estate value trends, credit score trends, delinquent loan trends, and industry and peer charge-off information. The qualitative loss factors for the commercial real estate loan portfolio take into consideration the composition of the portfolio along with industry and peer charge-off information and certain ACL ratios. As loans are classified or become delinquent, the qualitative loss factors increase for each respective loan category. The qualitative loss factors were derived by management based on a review of the historical performance of the respective loan portfolios and industry and peer information for those loan portfolios with no or limited historical loss experience, along with consideration of current economic conditions and the likely impact such conditions might have to the performance of the loan portfolio. Management utilizes the formula analysis model, along with analyzing and considering several other relevant internal and external data elements when evaluating the adequacy of the ACL. Such data elements include the trend and composition of delinquent loans and non-performing loans, trends in foreclosed property and short sale transactions and charge-off activity, the current status and trends of local and national employment levels, trends and current conditions in the housing markets, loan growth and concentrations, industry and peer charge-off and ACL information, and certain ACL ratios such as ACL to loans receivable, net and annualized historical losses. Since the Bank's loan portfolio is primarily concentrated in one- to four-family real estate, management monitors residential real estate market value trends in the Bank's local market areas and geographic sections of the U.S. by reference to various industry and market reports, economic releases and surveys, and management's general and specific knowledge of the real estate markets in which the Bank lends, in order to determine what impact, if any, such trends may have on the level of ACL. Reviewing these data elements assists management in evaluating the overall credit quality of the loan portfolio and the reasonableness of the ACL on an ongoing basis, and whether changes need to be made to the Bank's ACL methodology. Management seeks to apply the ACL methodology in a consistent manner; however, the methodology can be modified in response to changing conditions. Although management believes the ACL was at a level adequate to absorb inherent losses in the loan portfolio at September 30, 2017, the level of the ACL remains an estimate that is subject to significant judgment and short-term changes.
Federal Home Loan Bank Stock	As a member of FHLB Topeka, the Bank is required to acquire and hold shares of FHLB stock. The Bank's holding requirement varies based on the Bank's activities, primarily the Bank's outstanding borrowings, with FHLB. FHLB stock is carried at cost and is considered a restricted asset because it cannot be pledged as collateral or bought or sold on the open market and it also has certain redemption restrictions. Management conducts a quarterly evaluation to determine if any FHLB stock impairment exists. The quarterly impairment evaluation focuses primarily on the capital adequacy and liquidity of FHLB, while also considering the impact that legislative and regulatory developments may have on FHLB. Stock and cash dividends received on FHLB stock are reflected as dividend income in the consolidated statements of income.
Premises and Equipment	Land is carried at cost. Buildings, leasehold improvements, and furniture, fixtures and equipment are carried at cost less accumulated depreciation and leasehold amortization. Buildings, furniture, fixtures and equipment are depreciated over their estimated useful lives using the straight-line method. Buildings have an estimated useful life of 39 years. Structural components of the buildings generally have an estimated life of 15 years. Furniture, fixtures and equipment have an estimated useful life of three to seven years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the term of the respective leases, which is generally three to 15 years. The costs for major improvements and renovations are capitalized, while maintenance, repairs and minor improvements are charged to operating expenses as incurred. Gains and losses on dispositions are recorded as non-interest income or non-interest expense as incurred.
Income Taxes	The Company utilizes the asset and liability method of accounting for income taxes. Under this method, deferred income tax assets and liabilities are recognized for the tax consequences of temporary differences between the financial statement carrying amounts and the tax basis of existing assets and liabilities. Deferred income tax expense (benefit) represents the change in deferred income tax assets and liabilities excluding the tax effects of the change in net unrealized gain (loss) on AFS securities, interest rate swaps and changes in the market value of restricted stock between the grant date and vesting date. Income tax related penalties and interest are included in income tax expense in the consolidated statements of income. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. 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. Certain tax benefits attributable to stock options and restricted stock are credited to additional paid-in capital. To the extent that management considers it more likely than not that a deferred tax asset will not be recovered, a valuation allowance is recorded. All positive and negative evidence is reviewed in determining how much of a valuation allowance is recognized on a quarterly basis. Certain accounting literature prescribes a recognition threshold and measurement attribute for the financial statement recognition and measurement of an uncertain tax position taken, or expected to be taken, in a tax return. Interest and penalties related to unrecognized tax benefits are recognized in income tax expense in the consolidated statements of income. Accrued interest and penalties related to unrecognized tax benefits are included within the related tax liabilities line in the consolidated balance sheet.
Employee Stock Ownership Plan	The funds borrowed by the ESOP from the Company to purchase the Company's common stock are being repaid from dividends paid on unallocated ESOP shares and, if necessary, contributions by the Bank. The shares pledged as collateral are reported as a reduction of stockholders' equity at cost. As ESOP shares are committed to be released from collateral each quarter, the Company records compensation expense based on the average market price of the Company's stock during the quarter. Additionally, the shares become outstanding for EPS computations once they are committed to be released. The eligibility criteria for participation in the Company's ESOP is a minimum of one year of service, at least age 21, and at least 1,000 hours of employment in each plan year.
Stock-based Compensation	The Company has share-based plans under which stock options and restricted stock awards have been granted. Compensation expense is recognized over the service period of the share-based payment award. The Company utilizes a fair-value-based measurement method in accounting for the share-based payment transactions with employees, except for equity instruments held by the ESOP. The Company applies the modified prospective method in which compensation cost is recognized over the service period for all awards granted.
Borrowed Funds	The Bank has entered into repurchase agreements, which are sales of securities under agreements to repurchase, with approved counterparties. These agreements are recorded as financing transactions, and thereby reported as liabilities on the consolidated balance sheet, as the Bank maintains effective control over the transferred securities and the securities continue to be carried in the Bank's securities portfolio. The Bank has obtained borrowings from FHLB in the form of advances and a line of credit. Total FHLB borrowings are secured by certain qualifying loans pursuant to a blanket collateral agreement with FHLB and certain securities, as necessary. Additionally, the Bank is authorized to borrow from the Federal Reserve Bank's "discount window." The Company uses interest rate swaps as part of its interest rate risk management strategy to hedge the variable cash outflows associated with certain borrowings. Interest rate swaps are carried at fair value in the Company's consolidated financial statements. For interest rate swaps that are designated and qualify as cash flow hedges, the effective portion of changes in the fair value of such agreements are recorded in AOCI and are subsequently reclassified into interest expense in the period that interest on the borrowings affects earnings. The ineffective portion of the change in fair value of the interest rate swap is recognized directly in earnings. Effectiveness is assessed using regression analysis. At the inception of a hedge, the Company documents certain items, including the relationship between the hedging instrument and the hedged item, the risk management objective and the nature of the risk being hedged, a description of how effectiveness will be measured and an evaluation of hedged transaction effectiveness.
Segment Information	As a community-oriented financial institution, substantially all of the Bank's operations involve the delivery of loan and deposit products to customers. Management makes operating decisions and assesses performance based on an ongoing review of these community banking operations, which constitute the Company's only operating segment for financial reporting purposes.
Low Income Housing Partnerships	As part of the Bank's community reinvestment initiatives, the Bank invests in affordable housing limited partnerships ("low income housing partnerships") that make equity investments in affordable housing properties. The Bank is a limited partner in each partnership in which it invests. A separate, unrelated third party is the general partner. The Bank receives affordable housing tax credits and other tax benefits for these investments. Previously, the Bank accounted for low income housing partnerships using the equity method of accounting as two of the Bank's officers were involved in the operational management of the low income housing partnership investment group. Effective September 30, 2016, those two Bank officers discontinued their involvement in the operational management of the investment group. The Bank started using the proportional method of accounting for its low income housing partnership investments on October 1, 2016. See "Note 6. Low Income Housing Partnerships" for additional information.
Earnings Per Share	Basic EPS is computed by dividing income available to common stockholders by the weighted average number of shares outstanding for the period. Diluted EPS reflects the potential dilution that could occur if securities or other contracts to issue common stock (such as stock options) were exercised or resulted in the issuance of common stock. These potentially dilutive shares would then be included in the weighted average number of shares outstanding for the period using the treasury stock method. Shares issued and shares reacquired during any period are weighted for the portion of the period that they were outstanding. In computing both basic and diluted EPS, the weighted average number of common shares outstanding includes the ESOP shares previously allocated to participants and shares committed to be released for allocation to participants and restricted stock shares which have vested or have been allocated to participants. ESOP shares that have not been committed to be released are excluded from the computation of basic and diluted EPS. Unvested restricted stock awards contain nonforfeitable rights to dividends and are treated as participating securities in the computation of EPS pursuant to the two-class method.
Comprehensive Income	Comprehensive income consists of net income and other comprehensive income. Other comprehensive income includes unrealized gains and losses on AFS securities and changes in the accumulated gains/losses on effective cash flow hedging instruments, net of taxes.
Recent Accounting Pronouncements	In May 2014, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") 2014-09, Revenue from Contracts with Customers. The ASU, as amended, implements a common revenue standard that clarifies the principles for recognizing revenue. The core principle of the amended guidance is that an entity should recognize revenue to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Additionally, the amended guidance identifies specific steps an entity should apply in order to achieve this principle. The amended guidance requires entities to disclose both quantitative and qualitative information regarding contracts with customers. ASU 2014-09 will become effective for the Company on October 1, 2018. The majority of the Company's revenue is composed of interest income from loans and securities which are explicitly excluded from the amended ASU; therefore the amended ASU will likely not have a material impact to the Company's consolidated financial condition and results of operations, but it will likely result in expanded disclosures. The Company's evaluation of the amended ASU and its impact on components of non-interest income is ongoing. In January 2016, the FASB issued ASU 2016-01, Financial Instruments, Recognition and Measurement of Financial Assets and Liabilities. The ASU supersedes certain accounting guidance related to equity securities with readily determinable fair values and the related impairment assessment. An entity's equity investments that are accounted for under the equity method of accounting or result in consolidation of an investee are not included within the scope of this ASU. The ASU requires public business entities to utilize the exit price notation in determining fair value for financial instruments measured at amortized cost on the balance sheet. The ASU requires additional reporting in other comprehensive income for financial liabilities measured at fair value in accordance with the fair value option. The ASU also requires separate presentation of financial assets and financial liabilities by measurement category and form of financial asset on the balances or in the notes to the financial statements. ASU 2016-01 will become effective for the Company on October 1, 2018. The Company is currently evaluating the impact that this ASU may have on the Company's consolidated financial condition, results of operations and disclosures. In February 2016, the FASB issued ASU 2016-02, Leases. The ASU amends lease accounting guidance by requiring that lessees recognize the assets and liabilities arising from leases on the balance sheet. Additionally, the ASU requires entities to disclose both quantitative and qualitative information regarding their leasing activities. ASU 2016-02 will become effective for the Company on October 1, 2019. The Company is currently in the process of accumulating lease data and developing an inventory of leases. The Company expects to recognize right-of-use assets and lease liabilities for substantially all of its operating lease commitments based on the present value of unpaid lease payments as of the date of adoption. The Company is continuing to evaluate the impact this ASU may have on the Company's consolidated financial condition, results of operations and disclosures. In March 2016, the FASB issued ASU 2016-09, Compensation - Stock Compensation: Improvements to Employee Share-Based Payment Accounting. The ASU simplifies several aspects of the accounting for employee share-based payment transactions, including the accounting for income taxes, forfeitures, and statutory tax withholding requirements, along with simplifying the classification in the statement of cash flows. The ASU became effective for the Company on October 1, 2017. Upon adoption, the Company elected to account for forfeitures of stock-based compensation awards when they occur. The Company will recognize excess tax benefits and tax deficiencies in income tax expense on the consolidated statements of income and present them within operating activities on the consolidated statements of cash flows. This ASU did not have a material impact on the Company's consolidated financial condition or results of operations at the time of adoption. However, the impact of tax benefits and the timing of their recognition within income tax expense is unpredictable, as these benefits are recognized primarily as a result of stock options being exercised. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments. The ASU replaces the incurred loss impairment methodology in current GAAP, which requires credit losses to be recognized when it is probable that a loss has incurred, with a new impairment methodology. The new impairment methodology requires an entity to measure, at each reporting date, the expected credit losses of financial assets not measured at fair value, such as loans, HTM debt securities, and loan commitments, over their contractual lives. Under the new impairment methodology, expected credit losses will be measured at each reporting date based on historical experience, current conditions, and reasonable and supportable forecasts. Additionally, the ASU amends the current credit loss measurements for AFS debt securities. Credit losses related to AFS debt securities will be recorded through the ACL rather than as a direct write-down as per current GAAP. The ASU also requires enhanced disclosures related to credit quality and significant estimates and judgments used by management when estimating credit losses. The ASU will become effective for the Company on October 1, 2020. The Company has developed an implementation plan and is in the process of reviewing and assessing its processes and systems and identifying the necessary data to implement the ASU. In August 2017, the FASB issued ASU 2017-12, Derivatives and Hedging: Target Improvements to Accounting for Hedging Activities. The ASU amends the hedge accounting recognition and presentation requirements in current GAAP. The purpose of the ASU was to improve transparency of hedging relationships in the financial statements and to reduce the complexity of applying hedge accounting for preparers. The ASU will become effective for the Company on October 1, 2019. The Company is currently evaluating the effect of the ASU on the Company's consolidated financial condition, results of operations and disclosures.