Shareholders' Equity Earnings Per Share (Details) (USD $)	3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Apr. 12, 2013	Sep. 30, 2013 Stock Options	Sep. 30, 2012 Stock Options	Sep. 30, 2013 Stock Options	Sep. 30, 2012 Stock Options	Apr. 12, 2013 Citizens Republic Bancorp	Feb. 04, 2013 Citizens Republic Bancorp	Apr. 12, 2013 Citizens Republic Bancorp	Apr. 12, 2013 FirstMerit Citizens Republic Bancorp	Sep. 30, 2013 Series A Non-Cumulative Perpetual Preferred Stock	Feb. 04, 2013 Series A Non-Cumulative Perpetual Preferred Stock	Dec. 31, 2012 Series A Non-Cumulative Perpetual Preferred Stock	Sep. 30, 2012 Series A Non-Cumulative Perpetual Preferred Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 2,400,000	$ 1,800,000	$ 2,400,000	$ 1,800,000
Business Acquisition, Equity Interest Issued or Issuable, Share Conversion Ratio										1.37
FirstMerit shares of Common Stock issued for Citizens' shares										55,468,283
Average common and Common Stock equivalent shares outstanding	168,916,000	109,645,000	147,341,000	109,473,000		3,872,000	0	2,939,000	0
Net income	40,715,000	34,953,000	126,510,000	95,882,000
Dividends, Preferred Stock	1,469,000	0	3,868,000	0
Undistributed Earnings Allocated to Participating Securities	309,000	156,000	1,098,000	87,000
Net income available to common shareholders	38,937,000	34,797,000	121,544,000	95,795,000
Average Common Shares Outstanding Prior to Participating Share Adjustment	166,323,000	110,682,000	145,681,000	110,282,000
Incremental Common Shares Attributable to Participating Nonvested Shares with Non-forfeitable Dividend Rights	1,279,000	1,037,000	1,279,000	809,000
Weighted average number of common shares outstanding - basic	165,044,000	109,645,000	144,402,000	109,473,000
Earnings Per Share, Basic	$ 0.24	$ 0.32	$ 0.84	$ 0.88
Undistributed earnings allocated to unvested securities-Diluted EPS	105,000	176,000	485,000	342,000
Undistributed earnings allocated to participating securities-Diluted	105,000	176,000	485,000	342,000
Income used in diluted earnings per share calculation	38,937,000	34,797,000	121,544,000	95,795,000
Weighted average number of shares outstanding prior to participating share adjustment	166,323,000	110,682,000	145,681,000	110,282,000
Earnings Per Share, Diluted	$ 0.23	$ 0.32	$ 0.82	$ 0.88
Class of Warrant or Right, Number of Securities Called by Warrants or Rights												1,757,812.5	2,408,203
Exercise Price of Warrants or Rights					$ 18.69
5.875% Non-Cumulative Perpetual Preferred stock, Series A, authorized 115,000 shares	7,000,000		7,000,000											115,000		0	0
5.875% Non-Cumulative Perpetual Preferred stock, Series A, 100,000 outstanding														100,000	100,000	0	0
Interest rate of Non-Cumulative Perpetual Preferred Stock, Series A											5.875%