| **Nature of Operations and Summary of Significant Accounting Policies** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      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| **Accounting Policies [Abstract]**                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                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| Nature of Operations and Summary of Significant Accounting Policies     | NATURE OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIESThe accounting and reporting policies of the Corporation and the Bank, conform to accounting principles generally accepted in the United States of America and reporting practices followed by the banking industry. The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires 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 period. Actual results could differ from those estimates.  The Corporation is a financial holding company whose principal activity is the ownership and management of the Bank and operates in a single significant business segment. The Bank provides full banking services under an Indiana state-charter.  On April 1, 2016, the Board of Directors of the Bank adopted final resolutions approving the conversion of the Bank’s banking charter from a national association to an Indiana state-chartered bank.  The initial application to convert was filed with the Indiana DFI on February 9, 2016.  Between the date of initial application and adoption of the final resolutions by the Bank’s Board, the Indiana DFI and the FDIC conducted a joint exam of the Bank and its banking activities.  Final regulatory approval of the application was obtained at the meeting of the Members of the Indiana DFI on April 14, 2016.  The Bank filed official conversion documents effective April 15, 2016.  As a result of the conversion, the Indiana DFI became the Bank’s primary regulator and the FDIC became the Bank’s primary federal regulator.  Upon conversion, the Bank’s official name changed from “First Merchants Bank, National Association” to “First Merchants Bank.”  The Bank continues operating under the following trade names in certain geographic markets:  Lafayette Bank and Trust and First Merchants Private Wealth Advisors (each as a division of First Merchants Bank).  The conversion did not affect the Bank’s operations or customers in any way, and Bank customers continued to receive identical protection on deposits through the FDIC’s deposit insurance program.   The Bank generates commercial, mortgage, and consumer loans and receives deposits from customers located primarily in central and northern Indiana, northeast Illinois and central Ohio counties.  The Bank’s loans are generally secured by specific items of collateral, including real property, consumer assets and business assets.A brief description of current accounting practices and current valuation methodologies are presented below.CONSOLIDATION of the Corporation's financial statements include the accounts of the Corporation and all its subsidiaries, after elimination of all material intercompany transactions.BUSINESS COMBINATIONS are accounted for under the acquisition method of accounting.  Under the acquisition method, assets and liabilities of the business acquired are recorded at their estimated fair values as of the date of acquisition with any excess of the cost of the acquisition over the fair value of the net tangible and intangible assets acquired recorded as goodwill.  Results of operations of the acquired business are included in the income statement from the date of acquisition.AVAILABLE FOR SALE SECURITIES are recorded at fair value on a recurring basis with the unrealized gains and losses, net of applicable income taxes, recorded in other comprehensive income.  Realized gains and losses are recorded in earnings and the prior fair value adjustments are reclassified within stockholders' equity. Gains and losses on sales of securities are determined on the specific-identification method.  Amortization of premiums and accretion of discounts are recorded as interest income from securities. Available for sale and held to maturity securities are evaluated for OTTI at least on a quarterly basis, and more frequently when economic or market conditions warrant such an evaluation. The investment securities portfolio is evaluated for OTTI by segregating the portfolio into two general segments and applying the appropriate OTTI model. Investment securities are generally evaluated for OTTI under ASC 320, Investments - Debt and Equity Securities. However, certain purchased beneficial interest, including certain non-agency government-sponsored mortgage-backed securities, asset-backed securities and collateralized debt obligations are evaluated using the model outlined in ASC 325-10, Investments - Other.In determining OTTI under ASC 320, management considers many factors, including: (1) the length of time and the extent to which the fair value has been less than cost, (2) the financial condition and near term prospects of the issuer, (3) whether the market decline was affected by macroeconomic conditions, and (4) whether the Corporation has the intent to sell the debt security or more likely than not, will be required to sell the debt security before its anticipated recovery. The assessment of whether an other-than-temporary decline exists involves a high degree of subjectivity and judgment and is based on the information available to management at a point in time.When OTTI occurs, the amount of OTTI recognized in the income statement depends on whether the Corporation intends to sell the security or it is more likely than not that the Corporation will be required to sell the security before recovery of its amortized cost basis, less any recognized credit loss. If the intent is to sell, or it is more likely than not that the Corporation will be required to sell the security before recovery of its amortized cost basis, less any recognized credit loss, the OTTI shall be recognized in earnings equal to the entire difference between the investment’s amortized cost basis, less any recognized credit loss, and its fair value at the balance sheet date. If the intent is not to sell the security and it is not more likely than not that the Corporation will be required to sell the security before the recovery of its amortized cost basis less any recognized credit loss, the OTTI has been separated into the amount representing the credit loss and the amount related to all other factors. The amount of the total OTTI related to the credit loss is determined based on the present value of cash flows expected to be collected and is recognized in earnings. The amount of the total OTTI related to other factors is recognized in other comprehensive income, net of applicable income taxes. The previous amortized cost basis less the OTTI recognized in earnings becomes the new amortized cost basis of the investment. HELD TO MATURITY SECURITIES are classified as held to maturity when the Corporation has the positive intent and ability to hold the securities to maturity. Securities held to maturity are carried at amortized cost.  For held to maturity debt securities, the amount of an OTTI recorded in other comprehensive income for the noncredit portion of a previous other-than-temporary impairment is amortized prospectively over the remaining life of the security on the basis of the timing of future estimated cash flows of the security.LOANS HELD FOR SALE are carried at the principal amount outstanding.  The carrying amount approximates fair value due to the short duration between origination and the date of sale.LOANS held in the Corporation’s portfolio are carried at the principal amount outstanding, net of unearned income. Certain non-accrual,  substantially delinquent and renegotiated loans classified as troubled debt restructures may be considered to be impaired in accordance with ASC 310, Receivables.  Under ASC 310, a loan is impaired when, based on current information or events, it is probable that the Bank will be unable to collect all amounts due (principal and interest) according to the contractual terms of the loan agreement. Renegotiated consumer loans classified as troubled debt restructures are considered to be impaired.  In applying the provisions of ASC 310, the Corporation considers all other investments in one-to-four family residential loans and consumer installment loans to be homogeneous and therefore excluded from separate identification for evaluation of impairment. Interest income is accrued on the principal balances of loans. The accrual of interest is discontinued on a loan when, in management’s opinion, the borrower may be unable to meet payments as they become due. When the interest accrual is discontinued, all unpaid accrued interest is reversed against earnings when considered uncollectible. Interest income accrued in the prior year, if any, is charged to the allowance for loan losses.  Interest income is subsequently recognized only to the extent cash payments are received and the loan is returned to accruing status. Certain loan fees and direct costs are being deferred and amortized as an adjustment of yield on the loans.Impaired loans are carried at the fair value of collateral if the loan is collateral dependent, or the present value of estimated future cash flows using the loan’s existing rate. A portion of the allowance for loan losses is allocated to impaired loans if the value of such loans is deemed to be less than the unpaid balance. If these allocations cause the allowance for loan losses to increase, such increase is reported as a component of the provision for loan losses. Loan losses are charged against the allowance when management believes the uncollectability of the loan is confirmed. The valuation would be considered Level 3, consisting of appraisals of underlying collateral and discounted cash flow analysis.Loan commitments and letters-of-credit generally have short-term, variable-rate features and contain clauses which limit the Bank’s exposure to changes in customer credit quality. Accordingly, their carrying values, which are immaterial at the respective balance sheet dates, are reasonable estimates of fair value.LOANS ACQUIRED IN BUSINESS COMBINATIONS with evidence of credit deterioration since origination and for which it is probable that all contractually required payments will not be collected are considered to be purchased credit impaired.  Evidence of credit quality deterioration as of purchase dates may include information such as past-due and nonaccrual status, borrower credit scores and recent loan to value percentages.  Acquired credit-impaired loans are accounted for under the accounting guidance for loans and debt securities acquired with deteriorated credit quality (ASC 310-30).  These loans are initially measured at fair value based upon expected cash flows without anticipation of prepayments and includes estimated future credit losses expected to be incurred over the life of the loans.  As a result, related discounts are recognized subsequently through accretion based on the expected cash flows of the acquired loans.  For purposes of applying  ASC 310-30, loans acquired in business combinations are individually evaluated for the initial fair value measurement.  Accordingly, allowances for credit losses related to these loans are not carried over and recorded at the acquisition date. The difference between contractually required payments and the cash flows expected to be collected at acquisition is referred to as the nonaccretable portion of the fair value discount or premium.  The accretable portion of the fair value discount or premium is the difference between the expected cash flows and the net present value of expected cash flows, with such difference accreted into earnings over the term of the loans.  Acquired loans not accounted for under ASC 310-30 are accounted for under ASC 310-20, which allows the fair value adjustment to be accreted into income over the remaining life of the loans.ALLOWANCE FOR LOAN LOSSES is maintained to absorb losses inherent in the loan portfolio and is based on ongoing, quarterly assessments of the probable losses inherent in the loan portfolio. The allowance is increased by the provision for loan losses, which is charged against current operating results. Loan losses are charged against the allowance when management believes the uncollectability of a loan balance is confirmed. Subsequent recoveries, if any, are credited to the allowance. The Corporation’s strategy for credit risk management includes credit policies and underwriting criteria for all loans, as well as an overall credit limit for each customer significantly below legal lending limits. The strategy also emphasizes diversification on regional geographic and industry levels, regular credit quality reviews and management reviews of large credit exposures and loans experiencing deterioration of credit quality.The Corporation’s methodology for assessing the appropriateness of the allowance consists of three key elements – the determination of the appropriate reserves for impaired loans accounted for under ASC 310, probable losses estimated from historical loss rates, and probable losses resulting from economic, environmental, qualitative or other deterioration above and beyond what is reflected in the first two components of the allowance.Commercial relationships greater than $500,000 that exhibit probable or observed credit weaknesses are subject to individual review. Where appropriate, reserves are allocated to individual loans based on management’s estimate of the borrower’s ability to repay the loan given the availability of collateral, other sources of cash flow and legal options available to the Corporation. Included in the review of individual loans are those that are impaired as provided in ASC 310.  Any allowances for impaired loans are measured based on the fair value of the underlying collateral if collateral dependent or the present value of expected future cash flows discounted at the loan’s effective interest rate. The Corporation evaluates the collectability of principal when assessing the need for a loss accrual. Historical loss rates are applied to other commercial loans not subject to specific reserve allocations.The historical allocation for commercial loans graded pass are established by loan segments using loss rates based on the Corporation’s migration analysis. This migration analysis shows the loss rates for each segment of loans based on the loan grades at the beginning of the twelve month period. This loss rate is then applied to the current portfolio of loans in each respective loan segment.Homogenous loans, such as consumer installment and residential mortgage loans, are not individually risk graded. Reserves are established for each segment of loans using loss rates based on charge-offs for the same period as the migration analysis used for commercial loans.Historical loss allocations for commercial and consumer loans may be adjusted for significant factors that, in management’s judgment, reflect the impact of any current conditions on loss recognition. Factors which management considers in the analysis include the effects of the national and local economies, trends in loan growth and charge-off rates, changes in mix, concentration of loans in specific industries, asset quality trends (delinquencies, charge-offs and non-accrual loans), risk management and loan administration, changes in the internal lending policies and credit standards, examination results from bank regulatory agencies and the Corporation’s internal loan review.PENSION benefits are provided to the Corporation’s employees. Its accounting policies related to pensions and other post retirement benefits reflect the guidance in ASC 715, Compensation – Retirement Benefits. The Corporation does not consolidate the assets and liabilities associated with the pension plan. Instead, the Corporation recognizes the funded status of the plan in the consolidated balance sheets. The measurement of the funded status and the annual pension expense involves actuarial and economic assumptions. Various statistical and other factors, which attempt to anticipate future events, are used in calculating the expense and liabilities related to the plans. Key factors include assumptions on the expected rates of return on plan assets, discount rates, expected rates of salary increases and health care costs and trends. The Corporation considers market conditions, including changes in investment returns and interest rates in making these assumptions. The primary assumptions used in determining the Corporation’s pension and post retirement benefit obligations and related expenses are presented in NOTE 20. PENSION AND OTHER POST RETIREMENT BENEFIT PLANS of these Notes to Consolidated Financial Statements.PREMISES AND EQUIPMENT is carried at cost net of accumulated depreciation. Depreciation is computed using the straight-line and declining balance methods based on the estimated useful lives of the assets ranging from three to forty years. Maintenance and repairs are expensed as incurred, while major additions and improvements,  which extend the useful life, are capitalized. Gains and losses on dispositions are included in current operations.FEDERAL HOME LOAN BANK STOCK is a required investment for institutions that are members of the FHLB. The Bank is a member of the FHLB of Indianapolis.  Members are required to own a certain amount of stock based on the level of borrowings and other factors, and may invest in additional amounts.  FHLB stock is carried at cost, classified as a restricted security, and periodically evaluated for impairment based on ultimate recovery of par value.  Both cash and stock dividends are reported as income.INTANGIBLE ASSETS that are subject to amortization, including core deposit intangibles, are being amortized on both the straight-line and accelerated basis over two to twenty years. Intangible assets are periodically evaluated as to the recoverability of their carrying value.GOODWILL is maintained by applying the provisions of ASC 350, Intangibles – Goodwill and Other. For purchase acquisitions, the Corporation is required to record the assets acquired, including identified intangible assets, and the liabilities assumed at their fair value, which in many instances involves estimates based on third party valuations, such as appraisals, or internal valuations based on discounted cash flow analysis or other valuation techniques that may include estimates of attrition, inflation, asset growth rates or other relevant factors. In addition, the determination of the useful lives for which an intangible asset will be amortized is subjective.Under ASC 350, the Corporation is required to evaluate goodwill for impairment on an annual basis, as well as on an interim basis, if events or changes indicate that the asset may be impaired, indicating that the carrying value may not be recoverable. The Corporation has historically elected to test for goodwill impairment as of October 1 of each year and has determined that no impairment exists.BANK OWNED LIFE INSURANCE has been purchased on certain employees and directors of the Corporation.  The Corporation records the life insurance at the amount that can be realized under the insurance contract at the balance sheet date, which is the cash surrender value adjusted for other charges or amounts due that are probable at settlement.OTHER REAL ESTATE OWNED consists of assets acquired through, or in lieu of, loan foreclosure and are held for sale. They are initially recorded at fair value less cost to sell at the date of foreclosure, establishing a new cost basis.  Subsequent to foreclosure, valuations are periodically performed by management and the assets are carried at the lower of carrying amount or fair value less cost to sell.  Revenue and expenses from operations and changes in the valuation are included in other real estate owned and foreclosure expenses.  Other real estate owned also includes bank premises formerly, but no longer used for banking.  Bank premises are transferred at the lower of carrying value or fair value less cost to sell.DERIVATIVE INSTRUMENTS are carried at the fair value of the derivatives and reflects the estimated amounts that would have been received to terminate these contracts at the reporting date based upon pricing or valuation models applied to current market information.As part of the asset/liability management program, the Corporation will utilize, from time to time, interest rate floors, caps or swaps to reduce its sensitivity to interest rate fluctuations. These are derivative instruments, which are recorded as assets or liabilities in the consolidated balance sheets at fair value. Changes in the fair values of derivatives are reported in the consolidated statements of operations or AOCI depending on the use of the derivative and whether the instrument qualifies for hedge accounting. The key criterion for the hedge accounting is that the hedged relationship must be highly effective in achieving offsetting changes in those cash flows that are attributable to the hedged risk, both at inception of the hedge and on an ongoing basis.Derivatives that qualify for the hedge accounting treatment are designated as either: a hedge of the fair value of the recognized asset or liability or of an unrecognized firm commitment (a fair value hedge) or a hedge of a forecasted transaction or the variability of cash flows to be received or paid related to a recognized asset or liability (a cash flow hedge). To date, the Corporation has only entered into a cash flow hedge. For cash flow hedges, changes in the fair values of the derivative instruments are reported in AOCI to the extent the hedge is effective. The gains and losses on derivative instruments that are reported in AOCI are reflected in the consolidated statements of income in the periods in which the results of operations are impacted by the variability of the cash flows of the hedged item. Generally, net interest income is increased or decreased by amounts receivable or payable with respect to the derivatives, which qualify for hedge accounting. At inception of the hedge, the Corporation establishes the method it uses for assessing the effectiveness of the hedging derivative and the measurement approach for determining the ineffective aspect of the hedge. The ineffective portion of the hedge, if any, is recognized in the consolidated statements of income. The Corporation excludes the time value expiration of the hedge when measuring ineffectiveness.The Corporation offers interest rate derivative products (e.g. interest rate swaps) to certain of its high-quality commercial borrowers. This product allows customers to enter into an agreement with the Corporation to swap their variable rate loan to a fixed rate. These derivative products are designed to reduce, eliminate or modify the risk of changes in the borrower’s interest rate or market price risk. The extension of credit incurred through the execution of these derivative products is subject to the same approvals and rigorous underwriting standards as the related traditional credit product. The Corporation limits its risk exposure to these products by entering into a mirror-image, offsetting swap agreement with a separate, well-capitalized and rated counterparty previously approved by the Credit and Asset Liability Committee. By using these interest rate swap arrangements, the Corporation is also better insulated from the interest rate risk associated with underwriting fixed-rate loans. These derivative contracts are not designated against specific assets or liabilities under ASC 815, Derivatives and Hedging, and, therefore, do not qualify for hedge accounting. The derivatives are recorded on the balance sheet at fair value and changes in fair value of both the customer and the offsetting swap agreements are recorded (and essentially offset) in non-interest income. The fair value of the derivative instruments incorporates a consideration of credit risk (in accordance with ASC 820, Fair Value Measurements and Disclosures), resulting in some volatility in earnings each period.INCOME TAX in the consolidated statements of income includes deferred income tax provisions or benefits for all significant temporary differences in recognizing income and expenses for financial reporting and income tax purposes. The Corporation files consolidated income tax returns with its subsidiaries.  The Corporation is generally no longer subject to U.S. federal, state and local income tax examinations by tax authorities for tax years before 2014.The Corporation adopted the provisions of the ASC 740, Income Taxes, which prescribes a recognition threshold and measurement attribute for the financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. ASC 740 also provides guidance on derecognition, classification, interest and penalties, accounting in interim periods, disclosure and transition. As a result of the implementation of ASC 740, the Corporation did not identify any uncertain tax positions that it believes should be recognized in the financial statements.  The Corporation's policy is to recognize interest and penalties related to unrecognized tax benefits, if any, as a component of income tax expense. STOCK OPTION AND RESTRICTED STOCK AWARD PLANS are maintained by the Corporation. The compensation costs are recognized for stock options and restricted stock awards issued to employees and directors based on the fair value of these awards at the date of grant. A Black-Scholes model is utilized to estimate the fair value of stock options.  The market price of the Corporation’s common stock at the date of grant is used for restricted stock awards. Compensation expense is recognized over the appropriate service period, which is generally two or three years.TRANSFERS OF FINANCIAL ASSETS are accounted for as sales, when control over the assets has been surrendered.  Control over transferred assets is deemed to be surrendered when (1) the assets have been isolated from the Corporation and put presumptively beyond the reach of the transferor and its creditors, even in bankruptcy or other receivership, (2) the transferee obtains the right (free of conditions that constrain it from taking advantage of that right) to pledge or exchange the transferred assets and (3) the Corporation does not maintain effective control over the transferred assets through an agreement to repurchase them before their maturity or the ability to unilaterally cause the holder to return specific assets.NET INCOME PER SHARE is computed by dividing net income available to common shareholders by the weighted-average number of shares of common stock outstanding.  Diluted net income per share is computed by dividing net income available to common shareholders by the weighted-average number of shares of common stock outstanding, plus the dilutive effect of outstanding stock options and nonvested restricted stock.RECLASSIFICATIONS have been made to prior financial statements to conform to the current financial statement presentation. These reclassifications had no effect on net income. |

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| - Definition
The entire disclosure for the general note to the financial statements for the reporting entity which may include, descriptions of the basis of presentation, business description, significant accounting policies, consolidations, reclassifications, new pronouncements not yet adopted and changes in accounting principles.
+ References
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