Business Combinations Purchase Price Accounting Table (Details) (USD $)	6 Months Ended	0 Months Ended
	Jun. 30, 2013	Apr. 12, 2013 Citizens Republic Bancorp		Apr. 12, 2013 Core Deposits Citizens Republic Bancorp		Apr. 12, 2013 Customer Relationships Citizens Republic Bancorp
Business Acquisition [Line Items]
FirstMerit common stock shares issued for Citizens' shares		$ 55,468,283
Closing price per share of the Corporation's common stock on April 12, 2013		$ 16.68
Consideration from common stock conversion (1.37 ratio)	925,272,000	925,211,000
Cash paid to the Treasury for Citizens' TARP Preferred		355,371,000
Cash paid in lieu of fractional shares to the former Citizens' shareholders		61,000
Consideration from the warrant issued to the Treasury for Citizens' TARP warrant	3,000,000	3,000,000
Total purchase price		1,283,643,000
Cash and due from banks		544,380,000
Investment Securities		3,202,577,000
Loans		4,624,292,000
Premises and equipment		138,766,000
Intangible assets		84,774,000	[1]	70,774,000	[1]	14,000,000	[1]
Accrued interest receivable and other assets		678,910,000
Total assets		9,273,699,000
Deposits		7,276,754,000
Borrowings		908,824,000
Accrued taxes, expenses, and other liabilities		78,816,000
Total liabilities		8,264,394,000
Net identifiable assets acquired		1,009,305,000
Goodwill		$ 274,338,000

[1] Intangible assets consist of core deposit intangibles of $70.8 million and trust relationships of approximately $14.0 million. The useful life for which the core deposit intangible and the trust relationships are being amortized over is 15 and 12 years, respectively.