Business Combinations Recognized Identifable Assets Acquired and Liabilities Assumed (Details) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended		0 Months Ended		3 Months Ended		9 Months Ended				9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Apr. 12, 2013 Citizens Republic Bancorp		Sep. 30, 2013 Citizens Republic Bancorp	Jun. 30, 2013 Citizens Republic Bancorp	Sep. 30, 2013 Core Deposits	Sep. 30, 2013 Core Deposits Citizens Republic Bancorp	Apr. 12, 2013 Core Deposits Citizens Republic Bancorp		Sep. 30, 2013 Customer Relationships Citizens Republic Bancorp		Apr. 12, 2013 Customer Relationships Citizens Republic Bancorp
Business Acquisition [Line Items]
FirstMerit shares of Common Stock 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)			$ 925,211
Cash paid to the Treasury for Citizens' TARP Preferred			355,371
Cash paid in lieu of fractional shares to the former Citizens' shareholders			61
Consideration from the warrant issued to the Treasury for Citizens' TARP warrant	3,000	0	3,000
Business Combination, Consideration Transferred						1,283,643
Cash and due from banks			544,380
Investment Securities			3,202,577
Loans			4,624,293
Premises and equipment			138,536		200
Intangible assets			84,774	[1]					70,800	[1]	14,000	[1]	14,000	[1]
Accrued interest receivable and other assets			678,639
Total assets			9,273,199
Deposits			7,276,754
Borrowings			908,824
Accrued taxes, expenses, and other liabilities			78,816
Total liabilities			8,264,394
Net identifiable assets acquired			1,008,805
Goodwill			$ 274,838		$ 500
Finite-Lived Intangible Asset, Useful Life							10 years	15 years			12 years

[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 years and 12 years, respectively.