Summary of Significant Accounting Policies	9 Months Ended
Sep. 30, 2013
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Unless otherwise indicated, defined terms and abbreviations used herein have the meanings set forth in the accompanying Glossary of Acronyms and Abbreviations. Basis of Presentation - FirstMerit Corporation is a BHC whose principal asset is the Common Stock of its wholly-owned subsidiary, FirstMerit Bank, N. A. The Parent Company’s other subsidiaries include Citizens Savings Corporation of Stark County, FirstMerit Capital Trust I, FirstMerit Community Development Corporation, FirstMerit Risk Management, Inc., FMT, Inc., Citizens Funding Trust I and Citizens Michigan Statutory Trust I. All significant intercompany balances and transactions have been eliminated in consolidation. The accounting and reporting policies of the Corporation conform to GAAP and to general practices within the financial services industry. The consolidated balance sheet at December 31, 2012 has been derived from the audited consolidated financial statements at that date. The accompanying unaudited interim financial statements reflect all adjustments (consisting only of normally recurring adjustments) that are, in the opinion of Management, necessary for a fair statement of the results for the interim periods presented. Certain information and note disclosures normally included in financial statements prepared in accordance with GAAP have been omitted in accordance with the rules of the SEC. The unaudited consolidated financial statements of the Corporation as of September 30, 2013 and 2012 are not necessarily indicative of the results that may be achieved for the full fiscal year or for any future period. These unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes included in the Corporation’s Annual Report on Form 10-K for the year ended December 31, 2012 (the “2012 Form 10-K”). Certain reclassifications of prior year’s amounts have been made to conform to the current year presentation. Such reclassifications had no effect on net earnings or equity. As discussed in Note 2 (Business Combinations), on April 12, 2013 (the "Acquisition Date"), the Corporation completed the merger with Citizens, a Michigan corporation. As part of the merger with Citizens, the Corporation now has two active wholly owned trusts that were formed for the purpose of issuing securities and qualify as regulatory capital. Accordingly, the Corporation made a determination as to whether it should consolidate other entities or account for them on the equity method of accounting depending on whether it has a controlling financial interest in an entity of less than 100% of the voting interest of that entity to determine if it is a VIE. A VIE is a corporation, partnership, trust or any other legal structure used for business purposes that either (a) does not have equity investors with voting rights or (b) has equity investors that do not provide sufficient financial resources for the entity to support its activities. An entity that holds a variable interest in a VIE is required to consolidate the VIE if the entity is subject to a majority of the risk of loss from the VIE's activities, is entitled to receive a majority of the entity's residual returns or both. VIE treatment is considered for entities in which the total equity investment at risk is sufficient to enable the entity to finance itself independently and provides the equity holders with the obligation to absorb losses, the right to receive residual returns and the right to make financial and operating decisions. The two active wholly owned trusts acquired from the merger with Citizens are considered VIEs primarily due to the fact that the Corporation is not the primary beneficiary, and consequently, the trusts are not consolidated in the consolidated financial statements. Each of the two active trusts issued separate offerings of trust preferred securities to investors in 2006 and 2003, with respect to which there remain $48.7 million and $25.8 million in aggregate liquidation amount outstanding, respectively, as of September 30, 2013. The gross proceeds from the issuances were used to purchase junior subordinated deferrable interest debentures issued by Citizens, now assumed by the Corporation. These junior subordinated deferrable interest debentures are the sole asset of each trust. The trust preferred securities held by these entities qualify as Tier 1 capital and are classified as “long-term debt” on the Consolidated Balance Sheets, with the associated interest expense recorded in “long-term debt” on the Consolidated Statements of Comprehensive Income. The expected losses and residual returns of these entities are absorbed by the trust preferred stock holders, and consequently the Corporation is not exposed to loss related to these VIEs. There have been no other significant changes to the Corporation’s accounting policies as disclosed in the 2012 Form 10-K. In preparing these accompanying unaudited interim consolidated financial statements, subsequent events were evaluated through the time the consolidated financial statements were issued. Recently Adopted and Issued Accounting Standards FASB ASU 2013-11, Preparation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists. Prior to this ASU, GAAP did not include explicit guidance on the financial statement presentation of an unrecognized tax benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. This ASU requires, with limited exception, that an unrecognized tax benefit, or a portion of an unrecognized tax benefit, should be presented in the financial statements as a reduction to a DTA for a net operating loss carryforward, or similar tax loss, or a tax credit carryforward. The amendments in ASU 2013-11 are effective for fiscal years, and interim periods within those years, beginning after December 15, 2013, and should be applied prospectively to all unrecognized tax benefits that exist at the effective date. There is no impact to the Corporation's consolidated financial statements as the Corporation does not have an unrecognized tax benefit. FASB ASU 2013-10, Inclusion of the Fed Funds Effective Swap Rate (or Overnight Index Swap Rate) as a Benchmark Interest Rate for Hedge Accounting Purposes (a consensus of the Emerging Issues Task Force). The ASU permits the Fed Funds Effective Swap Rate (OIS) to be used as a benchmark interest rate for hedge accounting purposes, in addition to U.S. Treasury rates and LIBOR. The amendments also remove the restriction on using different benchmark rates for similar hedges. The ASU was effective prospectively for qualifying new or redesignated hedging relationships entered into on or after July 17, 2013. The ASU has no impact on the Corporation's current hedging relationships and, thus, no impact on the Corporation's consolidated financial statements. FASB ASU 2013-02, Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income. ASU 2013-02 amends the guidance on ASC 220-10 by requiring an entity to provide information about the amounts reclassified out of AOCI by component. In addition, an entity is required to present, either on the face of the statement where net income is presented or in the notes, significant amounts reclassified out of AOCI by the respective line items of net income but only if the amount reclassified is required under GAAP to be reclassified to net income in its entirety in the same reporting period. The Corporation adopted this ASU effective January 1, 2013 and has incorporated this new disclosure information into Note 14 (Changes and Reclassifications Out of Accumulated Other Comprehensive Income). FASB ASU 2012-06, Business Combinations: Subsequent Accounting for an Indemnification Asset Recognized at the Acquisition Date as a Result of a Government-Assisted Acquisition of a Financial Institution (a consensus of the FASB Emerging Issues Task Force). ASU 2012-06 amends the guidance in ASU 805-20 on the recognition of an indemnification asset as a result of a government-assisted acquisition of a financial institution when a subsequent change in the cash flows expected to be collected on the indemnification asset occurs (as a result of a change in cash flows expected to be collected on the assets subject to indemnification). A subsequent change in the measurement of the indemnification asset is to be accounted for on the same basis as the change in the assets subject to indemnification. Any amortization of changes in value are limited to the contractual term of the indemnification agreement (that is, the lesser of the term of the indemnification agreement and the remaining life of the indemnified assets). The Corporation adopted this ASU effective January 1, 2013, and the adoption did not have a significant impact on the Corporation's consolidated financial statements. FASB ASU 2011-11, Disclosures about Offsetting Assets and Liabilities. ASU 2011-11 amends the guidance in ASC 210, Balance Sheet, to require an entity to disclose information about offsetting and related arrangements to enable users of an entity's financial statements to evaluate the effect or potential effect of netting arrangements. In January 2013, the FASB issued ASU 2013-11, Clarifying the Scope of Disclosures about Offsetting Assets and Liabilities, which more narrowly defined the scope of financial instruments to only derivatives, repurchase and reverse purchase agreements, and securities borrowing and lending transactions. The Corporation adopted these ASUs as of January 1, 2013, and the adoption did not have a significant impact on the Corporation's consolidated financial statements. The newly required disclosures are incorporated into Note 9 (Derivatives and Hedging Activities).

Summary of Significant Accounting Policies

Unless otherwise indicated, defined terms and abbreviations used herein have the meanings set forth in the accompanying Glossary of Acronyms and Abbreviations.

Basis of Presentation - FirstMerit Corporation is a BHC whose principal asset is the Common Stock of its wholly-owned subsidiary, FirstMerit Bank, N. A. The Parent Company’s other subsidiaries include Citizens Savings Corporation of Stark County, FirstMerit Capital Trust I, FirstMerit Community Development Corporation, FirstMerit Risk Management, Inc., FMT, Inc., Citizens Funding Trust I and Citizens Michigan Statutory Trust I. All significant intercompany balances and transactions have been eliminated in consolidation.

The accounting and reporting policies of the Corporation conform to GAAP and to general practices within the financial services industry.

The consolidated balance sheet at December 31, 2012 has been derived from the audited consolidated financial statements at that date. The accompanying unaudited interim financial statements reflect all adjustments (consisting only of normally recurring adjustments) that are, in the opinion of Management, necessary for a fair statement of the results for the interim periods presented. Certain information and note disclosures normally included in financial statements prepared in accordance with GAAP have been omitted in accordance with the rules of the SEC. The unaudited consolidated financial statements of the Corporation as of September 30, 2013 and 2012 are not necessarily indicative of the results that may be achieved for the full fiscal year or for any future period. These unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes included in the Corporation’s Annual Report on Form 10-K for the year ended December 31, 2012 (the “2012 Form 10-K”). Certain reclassifications of prior year’s amounts have been made to conform to the current year presentation. Such reclassifications had no effect on net earnings or equity.

As discussed in Note 2 (Business Combinations), on April 12, 2013 (the "Acquisition Date"), the Corporation completed the merger with Citizens, a Michigan corporation. As part of the merger with Citizens, the Corporation now has two active wholly owned trusts that were formed for the purpose of issuing securities and qualify as regulatory capital. Accordingly, the Corporation made a determination as to whether it should consolidate other entities or account for them on the equity method of accounting depending on whether it has a controlling financial interest in an entity of less than 100% of the voting interest of that entity to determine if it is a VIE. A VIE is a corporation, partnership, trust or any other legal structure used for business purposes that either (a) does not have equity investors with voting rights or (b) has equity investors that do not provide sufficient financial resources for the entity to support its activities. An entity that holds a variable interest in a VIE is required to consolidate the VIE if the entity is subject to a majority of the risk of loss from the VIE's activities, is entitled to receive a majority of the entity's residual returns or both. VIE treatment is considered for entities in which the total equity investment at risk is sufficient to enable the entity to finance itself independently and provides the equity holders with the obligation to absorb losses, the right to receive residual returns and the right to make financial and operating decisions.

The two active wholly owned trusts acquired from the merger with Citizens are considered VIEs primarily due to the fact that the Corporation is not the primary beneficiary, and consequently, the trusts are not consolidated in the consolidated financial statements. Each of the two active trusts issued separate offerings of trust preferred securities to investors in 2006 and 2003, with respect to which there remain $48.7 million and $25.8 million in aggregate liquidation amount outstanding, respectively, as of September 30, 2013. The gross proceeds from the issuances were used to purchase junior subordinated deferrable interest debentures issued by Citizens, now assumed by the Corporation. These junior subordinated deferrable interest debentures are the sole asset of each trust. The trust preferred securities held by these entities qualify as Tier 1 capital and are classified as “long-term debt” on the Consolidated Balance Sheets, with the associated interest expense recorded in “long-term debt” on the Consolidated Statements of Comprehensive Income. The expected losses and residual returns of these entities are absorbed by the trust preferred stock holders, and consequently the Corporation is not exposed to loss related to these VIEs.

There have been no other significant changes to the Corporation’s accounting policies as disclosed in the 2012 Form 10-K.

In preparing these accompanying unaudited interim consolidated financial statements, subsequent events were evaluated through the time the consolidated financial statements were issued.

Recently Adopted and Issued Accounting Standards

FASB ASU 2013-11, Preparation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists. Prior to this ASU, GAAP did not include explicit guidance on the financial statement presentation of an unrecognized tax benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. This ASU requires, with limited exception, that an unrecognized tax benefit, or a portion of an unrecognized tax benefit, should be presented in the financial statements as a reduction to a DTA for a net operating loss carryforward, or similar tax loss, or a tax credit carryforward. The amendments in ASU 2013-11 are effective for fiscal years, and interim periods within those years, beginning after December 15, 2013, and should be applied prospectively to all unrecognized tax benefits that exist at the effective date. There is no impact to the Corporation's consolidated financial statements as the Corporation does not have an unrecognized tax benefit.

FASB ASU 2013-10, Inclusion of the Fed Funds Effective Swap Rate (or Overnight Index Swap Rate) as a Benchmark Interest Rate for Hedge Accounting Purposes (a consensus of the Emerging Issues Task Force). The ASU permits the Fed Funds Effective Swap Rate (OIS) to be used as a benchmark interest rate for hedge accounting purposes, in addition to U.S. Treasury rates and LIBOR. The amendments also remove the restriction on using different benchmark rates for similar hedges. The ASU was effective prospectively for qualifying new or redesignated hedging relationships entered into on or after July 17, 2013. The ASU has no impact on the Corporation's current hedging relationships and, thus, no impact on the Corporation's consolidated financial statements.

FASB ASU 2013-02, Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income. ASU 2013-02 amends the guidance on ASC 220-10 by requiring an entity to provide information about the amounts reclassified out of AOCI by component. In addition, an entity is required to present, either on the face of the statement where net income is presented or in the notes, significant amounts reclassified out of AOCI by the respective line items of net income but only if the amount reclassified is required under GAAP to be reclassified to net income in its entirety in the same reporting period. The Corporation adopted this ASU effective January 1, 2013 and has incorporated this new disclosure information into Note 14 (Changes and Reclassifications Out of Accumulated Other Comprehensive Income).

FASB ASU 2012-06, Business Combinations: Subsequent Accounting for an Indemnification Asset Recognized at the Acquisition Date as a Result of a Government-Assisted Acquisition of a Financial Institution (a consensus of the FASB Emerging Issues Task Force). ASU 2012-06 amends the guidance in ASU 805-20 on the recognition of an indemnification asset as a result of a government-assisted acquisition of a financial institution when a subsequent change in the cash flows expected to be collected on the indemnification asset occurs (as a result of a change in cash flows expected to be collected on the assets subject to indemnification). A subsequent change in the measurement of the indemnification asset is to be accounted for on the same basis as the change in the assets subject to indemnification. Any amortization of changes in value are limited to the contractual term of the indemnification agreement (that is, the lesser of the term of the indemnification agreement and the remaining life of the indemnified assets). The Corporation adopted this ASU effective January 1, 2013, and the adoption did not have a significant impact on the Corporation's consolidated financial statements.

FASB ASU 2011-11, Disclosures about Offsetting Assets and Liabilities. ASU 2011-11 amends the guidance in ASC 210, Balance Sheet, to require an entity to disclose information about offsetting and related arrangements to enable users of an entity's financial statements to evaluate the effect or potential effect of netting arrangements. In January 2013, the FASB issued ASU 2013-11, Clarifying the Scope of Disclosures about Offsetting Assets and Liabilities, which more narrowly defined the scope of financial instruments to only derivatives, repurchase and reverse purchase agreements, and securities borrowing and lending transactions. The Corporation adopted these ASUs as of January 1, 2013, and the adoption did not have a significant impact on the Corporation's consolidated financial statements. The newly required disclosures are incorporated into Note 9 (Derivatives and Hedging Activities).