Summary of Significant Accounting Policies	3 Months Ended
Mar. 31, 2013
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Basis of Presentation - FirstMerit Corporation (the “Parent Company”) is a bank holding company whose principal asset is the common stock of its wholly-owned subsidiary, FirstMerit Bank, N. A. (the “Bank”). 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., and FMT, Inc. All significant intercompany balances and transactions have been eliminated in consolidation. The accounting and reporting policies of FirstMerit Corporation and its subsidiaries (the “Corporation”) conform to U.S. generally accepted accounting principles (“U.S. 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 FirstMerit Corporation’s Management (“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 U.S. GAAP have been omitted in accordance with the rules of the Securities and Exchange Commission (“SEC”). The unaudited consolidated financial statements of the Corporation as of March 31, 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. There have been no 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 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. The amendments are effective for annual reporting periods beginning on or after January 1, 2013, with retrospective application to the disclosures of all comparative periods presented. ASU 2011-11 did not have a significant impact on the corporations derivatives, repurchase agreements, reverse repurchase agreements, and securities lending and borrowing transactions on the consolidated financial statements. The newly required disclosures are incorporated into Note 9 (Derivatives and Hedging Activities). 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 amendments in ASU 2012-06 are effective for fiscal years, and interim periods within those years, beginning on or after December 15, 2012. The Corporation currently applies the accounting as described within ASU 2012-06; therefore, ASU 2012-06 will not have an impact on its 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 accumulated other comprehensive income 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 accumulated other comprehensive income by the respective line items of net income but only if the amount reclassified is required under U.S. GAAP to be reclassified to net income in its entirety in the same reporting period. The objective of the update is to improve the reporting of reclassifications out of accumulated other comprehensive income. The amendments in ASU 2013-02 are effective prospectively for reporting periods beginning after December 15, 2012. The Corporation has incorporated this new disclosure information into Note 14 (Changes and Reclassifications Out of Accumulated Other Comprehensive Income).

Basis of Presentation - FirstMerit Corporation (the “Parent Company”) is a bank holding company whose principal asset is the common stock of its wholly-owned subsidiary, FirstMerit Bank, N. A. (the “Bank”). 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., and FMT, Inc. All significant intercompany balances and transactions have been eliminated in consolidation.

The accounting and reporting policies of FirstMerit Corporation and its subsidiaries (the “Corporation”) conform to U.S. generally accepted accounting principles (“U.S. 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 FirstMerit Corporation’s Management (“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 U.S. GAAP have been omitted in accordance with the rules of the Securities and Exchange Commission (“SEC”). The unaudited consolidated financial statements of the Corporation as of March 31, 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.

There have been no 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 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. The amendments are effective for annual reporting periods beginning on or after January 1, 2013, with retrospective application to the disclosures of all comparative periods presented. ASU 2011-11 did not have a significant impact on the corporations derivatives, repurchase agreements, reverse repurchase agreements, and securities lending and borrowing transactions on the consolidated financial statements. The newly required disclosures are incorporated into Note 9 (Derivatives and Hedging Activities).

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 amendments in ASU 2012-06 are effective for fiscal years, and interim periods within those years, beginning on or after December 15, 2012. The Corporation currently applies the accounting as described within ASU 2012-06; therefore, ASU 2012-06 will not have an impact on its 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 accumulated other comprehensive income 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 accumulated other comprehensive income by the respective line items of net income but only if the amount reclassified is required under U.S. GAAP to be reclassified to net income in its entirety in the same reporting period. The objective of the update is to improve the reporting of reclassifications out of accumulated other comprehensive income. The amendments in ASU 2013-02 are effective prospectively for reporting periods beginning after December 15, 2012. The Corporation has incorporated this new disclosure information into Note 14 (Changes and Reclassifications Out of Accumulated Other Comprehensive Income).