Business Combinations (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Mar. 31, 2011 Midwest Bank and Trust Company [Member]	May 14, 2010 Midwest Bank and Trust Company [Member]	Jun. 30, 2011 George Washington Savings Bank [Member]	Feb. 19, 2010 George Washington Savings Bank [Member]	Feb. 19, 2010 Acquired Book Value [Member] First Bank Branches [Member]	Feb. 19, 2010 Fair Value Adjustments [Member] First Bank Branches [Member]	Dec. 31, 2011 Fair Value Adjustments [Member] Midwest Bank and Trust Company [Member]	Dec. 31, 2011 Fair Value Adjustments [Member] George Washington Savings Bank [Member]	Feb. 19, 2010 As Recorded by First Merit Bank, N.A. [Member] First Bank Branches [Member]	Dec. 31, 2011 As Recorded by First Merit Bank, N.A. [Member] Midwest Bank and Trust Company [Member]	Dec. 31, 2011 As Recorded by First Merit Bank, N.A. [Member] George Washington Savings Bank [Member]	Dec. 31, 2011 Recorded By FDIC [Member] Midwest Bank and Trust Company [Member]	Dec. 31, 2011 Recorded By FDIC [Member] George Washington Savings Bank [Member]
ASSETS
Cash and due from banks								$ 3,725	$ 0	$ 0	$ 0	$ 3,725	$ 279,352	$ 57,984	$ 279,352	$ 57,984
Investment securities										(977)	0		564,233	15,410	565,210	15,410
Loans								301,236	(25,624)			275,612
Covered loans
Commercial loan										(317,526)	(117,879)		1,522,475	136,613	1,840,001	254,492
Consumer loans										(53,742)			258,389		312,131
Mortgage Loan											(2,860)			24,358		27,218
Installment loan											(7,298)			16,780		24,078
Total covered loans					1,800,000		177,800			(371,268)	(128,037)		1,780,864	177,751	2,152,132	305,788
Allowance for loan losses										5,465			0		(5,465)
Accrued interest										(5,436)			0		5,436
Loss share receivable					260,700	88,700				260,730	88,694		260,730	88,694	0	0
Total covered loans and loss share receivable										(110,509)	(39,343)		2,041,594	266,445	2,152,103	305,788
Core deposit intangible								0	3,154	7,433	962	3,154	7,433	962	0	0
Covered other real estate										(1,165)	(7,561)		26,155	11,460	27,320	19,021
Loss share receivable-other real estate										2,196	11,339		2,196	11,339	0	0
Premises and equipment								22,992	18,963			41,955
Goodwill	460,044	460,044	139,598	272,100				0	48,347	272,099		48,347	272,099		0
Other assets								941	3,115	19,054	0	4,056	28,892	5,680	9,838	5,680
Total assets acquired								328,894	47,955	188,131	(34,603)	376,849	3,221,954	369,280	3,033,823	403,883
Deposits:
Non Interest Bearing Deposit Accounts											0			54,242		54,242
Savings deposits										0	0		748,681	62,737	748,681	62,737
Time deposits										9,125	4,921		1,509,038	283,676	1,499,913	278,755
Deposits								1,199,279	7,134	9,125	4,921	1,206,413	2,257,719	400,655	2,248,594	395,734
Borrowings										83,241			723,045		639,804
FDIC Liability										6,256			6,256		0
Accrued expenses and other liabilities								4,192	(1,271)	0	5,191	2,921	7,395	7,760	7,395	2,569
Total liabilities assumed								$ 1,203,471	$ 5,863	$ 98,622	$ 10,112	$ 1,209,334	$ 2,994,415	$ 408,415	$ 2,895,793	$ 398,303