Business Combinations (Details) (USD $)	3 Months Ended		9 Months Ended				0 Months Ended		3 Months Ended		9 Months Ended			9 Months Ended						3 Months Ended	6 Months Ended
	Sep. 30, 2013 Branch	Sep. 30, 2012	Sep. 30, 2013 Branch	Sep. 30, 2012	Apr. 12, 2013	Dec. 31, 2012	Apr. 15, 2013 Citizens Republic Bancorp	Apr. 12, 2013 Citizens Republic Bancorp	Sep. 30, 2013 Citizens Republic Bancorp Branch	Jun. 30, 2013 Citizens Republic Bancorp	Sep. 30, 2013 Citizens Republic Bancorp Branch	Feb. 04, 2013 Citizens Republic Bancorp	Sep. 12, 2012 Citizens Republic Bancorp	Sep. 30, 2013 Citizens Republic Bancorp Junior Subordinated Debt	Sep. 30, 2013 Citizens Republic Bancorp Junior Subordinated Debt Minimum	Sep. 30, 2013 Citizens Republic Bancorp Junior Subordinated Debt Maximum	Apr. 12, 2013 FirstMerit Citizens Republic Bancorp	Apr. 12, 2013 Citizens Republic Bancorp	Apr. 12, 2013 Acquired Loans	Sep. 30, 2013 Michigan and Wisconsin	Sep. 30, 2013 Michigan and Wisconsin
Business Combinations [Line Items]
Loans								$ 4,624,293,000											$ 4,624,293,000
Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed Loans									200,000		200,000
Assets	24,134,986,000	14,628,843,000	24,134,986,000	14,628,843,000		14,913,012,000							9,600,000,000
Accumulated but unpaid interest and dividends related to Citizens TARP Preferred Stock								55,400,000
Number of Branches	412		412						219		219
Business Acquisition, Equity Interest Issued or Issuable, Share Conversion Ratio								1.37
FirstMerit shares of Common Stock issued for Citizens' shares								55,468,283
Amount of Citizens TARP Preferred Stock								300,000,000
Debt Instrument, Face Amount												250,000,000
Interest Rate subordinated notes												4.35%
Total gross proceeds from issuance of depositary shares												100,000,000
Interest rate of Non-Cumulative Perpetual Preferred Stock, Series A												5.875%
Class of Warrant or Right, Number of Securities Called by Warrants or Rights																	2,408,203	1,757,812.5
Goodwill								274,838,000	500,000
Interest Income, Operating																				164,400,000
Net income	40,715,000	34,953,000	126,510,000	95,882,000																	31,600,000
Business Acquisition, Purchase Price Allocation, deferred tax asset								312,900,000
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deferred Tax Asset, Net								345,100,000	300,000		300,000
Business Acquisition, Purchase Price Allocation, Deferred Tax Liabilities, Noncurrent								51,300,000
Deferred Tax Assets, Net								261,600,000
Net Operating Loss and Tax Credit Carryforwards included in Acquired Deferred Tax Liability								224,700,000
Business combination, deferred tax asset fair market value adjustment								83,500,000
Business Combination, Acquisition Related Costs									33,400,000		69,100,000
Contractual required payments receivable																			5,107,315,000
Nonaccretable difference																			(130,096,000)
Investment Securities								3,202,577,000
Amount of securities sold subsequent to acquisition										2,200,000,000
Repurchase of securities subsequent to acquisition										1,500,000,000
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Federal Home Loan Bank Advances								719,300,000
Repayments of Federal Home Loan Bank Borrowings							591,900,000
Prepayment penalties on federal home loan bank advances							60,600,000
Federal home loan bank advances retained									60,000,000		60,000,000
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Subordinated Debt								74,500,000
Debt Instrument, Description of Variable Rate Basis														LIBOR
Debt Instrument, Basis Spread on Variable Rate															3.10%	11.75%
Debt Instrument, Interest Rate, Stated Percentage														7.50%
Business Combination Recognized Identifiable Assets Acquired And Liabilities Long Term Repurchase Agreements								115,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment								138,536,000	200,000		200,000
Business Combination Recognized Identifiable Assets Acquired And Liabilities, Accrued Interest and Other Assets									400,000		400,000
Exercise Price of Warrants or Rights					$ 18.69