Subsequent Events (unaudited)	3 Months Ended
Mar. 31, 2013
Subsequent Events [Abstract]
Subsequent Events (unaudited)	15. Subsequent Events On April 12, 2013, the Corporation completed the Merger with Citizens which resulted in Citizens common stock being converted into the right to receive 1.37 shares of the Corporation's common stock (except that any shares of Citizens common stock that were owned by Citizens, the Corporation or any of their respective subsidiaries, other than in a fiduciary capacity, were canceled without any consideration therefor). Each outstanding option to acquire, and each outstanding equity award relating to, one share of Citizens' common stock was converted into an option to acquire, or an equity award relating to, 1.37 shares of the Corporation's common stock, as applicable. The conversion of Citizens' common stock into the Corporation's common stock resulted in the Corporation issuing approximately 55 million shares of its common stock. Immediately following the Merger, Citizens Bank, a Michigan chartered bank and wholly owned subsidiary of Citizens, merged with and into FirstMerit Bank, N.A., a national banking association and wholly owned subsidiary of FirstMerit, with FirstMerit Bank, N.A. surviving the merger and continuing its corporate existence under the name “FirstMerit Bank, N.A.” The Merger Agreement provided that upon completion of the Merger, the Corporation increase its board of directors by two directors. The new directorships were filled with current members of the Citizens board. Effective April 12, 2013, the Corporation purchased the Fixed Rate Cumulative Perpetual Preferred Stock, Series A, of Citizens issued to the United States Treasury as part of the Troubled Assets Relief Program (the "Citizens TARP Preferred") in the amount of $300 million plus accumulated but unpaid dividends and interest of approximately $55.4 million. In addition, effective April 12, 2013, a warrant to purchase 1,757,812.5 shares of Citizens' common stock that had been issued to the Treasury on December 12, 2008 as part of Citizens' participation in the Treasury's Capital Purchase Program, was converted into a warrant to purchase 2,408,203 shares of FirstMerit common stock. The Corporation used the net proceeds from its February 4, 2013 public offerings to repurchase the Citizens TARP Preferred and pay all accrued, accumulated and unpaid dividends and interest. The Corporation's public offerings consisted of $250 million aggregate principal amount of subordinated notes due February 4, 2023 bearing interest at an annual rate of 4.35% payable semi-annually in arrears on February 4 and August 4 of each year and 4,000,000 depositary shares (each representing a 1/40th interest in a share of the Corporation's Series A Non-Cumulative Perpetual Preferred Stock) with dividends payable quarterly in arrears on the 4th day of February, May, August and November, beginning May 4, 2013, which resulted in gross proceeds of $100 million. The following components make up the consideration for the Merger transaction: approximately $930 million in converted common stock, $355 million paid to the Treasury to purchase the Citizens TARP Preferred and approximately $3 million in a warrant issued to the Treasury to purchase the Corporation's common stock in conjunction with the purchase of the Citizens TARP Preferred. The Citizens acquisition will be accounted for using the purchase acquisition method of accounting and accordingly, assets acquired, liabilities assumed and consideration exchanged will be recorded at estimated fair value on the acquisition date. The Corporation is in the process of determining the preliminary fair values which are subject to refinement for up to one year after the closing date of the acquisition as additional information relative to closing date fair values becomes available. Any resulting goodwill will not be deductible for income tax purposes as the acquisition is accounted for as a tax-free exchange for tax purposes. The results of operations acquired in the Citizens transaction will be included in the Corporation's financial results beginning on April 13, 2013. Management's strategy to de-leverage the newly acquired Citizens' balance sheet resulted in the Corporation repaying on April 15, 2013 approximately $591.0 million in principal of Federal Home Loan advances. Management's strategy to re-balance the investment portfolio acquired in the Citizens merger resulted in the sale of approximately $2.1 billion in agency MBS, agency CMOs, municipal securities and private label MBS investments in April 2013. Management intends on repurchasing $1.4 billion of agency MBS and CMO securities throughout the second quarter of 2013. In preparing these financial statements, subsequent events were evaluated through the time the financial statements were issued. Financial statements are considered issued when they are widely distributed to all shareholders and other financial statement users, or filed with the Securities and Exchange Commission. In accordance with applicable accounting standards, all material subsequent events have been either recognized in the financial statements or disclosed in the notes to the financial statements.

15. Subsequent Events

On April 12, 2013, the Corporation completed the Merger with Citizens which resulted in Citizens common stock being converted into the right to receive 1.37 shares of the Corporation's common stock (except that any shares of Citizens common stock that were owned by Citizens, the Corporation or any of their respective subsidiaries, other than in a fiduciary capacity, were canceled without any consideration therefor). Each outstanding option to acquire, and each outstanding equity award relating to, one share of Citizens' common stock was converted into an option to acquire, or an equity award relating to, 1.37 shares of the Corporation's common stock, as applicable. The conversion of Citizens' common stock into the Corporation's common stock resulted in the Corporation issuing approximately 55 million shares of its common stock.

Immediately following the Merger, Citizens Bank, a Michigan chartered bank and wholly owned subsidiary of Citizens, merged with and into FirstMerit Bank, N.A., a national banking association and wholly owned subsidiary of FirstMerit, with FirstMerit Bank, N.A. surviving the merger and continuing its corporate existence under the name “FirstMerit Bank, N.A.”

The Merger Agreement provided that upon completion of the Merger, the Corporation increase its board of directors by two directors. The new directorships were filled with current members of the Citizens board.

Effective April 12, 2013, the Corporation purchased the Fixed Rate Cumulative Perpetual Preferred Stock, Series A, of Citizens issued to the United States Treasury as part of the Troubled Assets Relief Program (the "Citizens TARP Preferred") in the amount of $300 million plus accumulated but unpaid dividends and interest of approximately $55.4 million. In addition, effective April 12, 2013, a warrant to purchase 1,757,812.5 shares of Citizens' common stock that had been issued to the Treasury on December 12, 2008 as part of Citizens' participation in the Treasury's Capital Purchase Program, was converted into a warrant to purchase 2,408,203 shares of FirstMerit common stock. The Corporation used the net proceeds from its February 4, 2013 public offerings to repurchase the Citizens TARP Preferred and pay all accrued, accumulated and unpaid dividends and interest. The Corporation's public offerings consisted of $250 million aggregate principal amount of subordinated notes due February 4, 2023 bearing interest at an annual rate of 4.35% payable semi-annually in arrears on February 4 and August 4 of each year and 4,000,000 depositary shares (each representing a 1/40th interest in a share of the Corporation's Series A Non-Cumulative Perpetual Preferred Stock) with dividends payable quarterly in arrears on the 4th day of February, May, August and November, beginning May 4, 2013, which resulted in gross proceeds of $100 million.

The following components make up the consideration for the Merger transaction: approximately $930 million in converted common stock, $355 million paid to the Treasury to purchase the Citizens TARP Preferred and approximately $3 million in a warrant issued to the Treasury to purchase the Corporation's common stock in conjunction with the purchase of the Citizens TARP Preferred.

The Citizens acquisition will be accounted for using the purchase acquisition method of accounting and accordingly, assets acquired, liabilities assumed and consideration exchanged will be recorded at estimated fair value on the acquisition date. The Corporation is in the process of determining the preliminary fair values which are subject to refinement for up to one year after the closing date of the acquisition as additional information relative to closing date fair values becomes available. Any resulting goodwill will not be deductible for income

tax purposes as the acquisition is accounted for as a tax-free exchange for tax purposes. The results of operations acquired in the Citizens transaction will be included in the Corporation's financial results beginning on April 13, 2013.

Management's strategy to de-leverage the newly acquired Citizens' balance sheet resulted in the Corporation repaying on April 15, 2013 approximately $591.0 million in principal of Federal Home Loan advances. Management's strategy to re-balance the investment portfolio acquired in the Citizens merger resulted in the sale of approximately $2.1 billion in agency MBS, agency CMOs, municipal securities and private label MBS investments in April 2013. Management intends on repurchasing $1.4 billion of agency MBS and CMO securities throughout the second quarter of 2013.

In preparing these financial statements, subsequent events were evaluated through the time the financial statements were issued. Financial statements are considered issued when they are widely distributed to all shareholders and other financial statement users, or filed with the Securities and Exchange Commission. In accordance with applicable accounting standards, all material subsequent events have been either recognized in the financial statements or disclosed in the notes to the financial statements.