Borrowings - Narrative (Details) (USD $)	0 Months Ended	12 Months Ended				3 Months Ended			0 Months Ended						12 Months Ended						0 Months Ended
	Sep. 09, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Subordinated Debentures and Term Loans	Dec. 31, 2011 Subordinated Debentures and Term Loans	Dec. 31, 2012 Subordinated Debt First Merchant Capital Trust II	Sep. 15, 2012 Subordinated Debt First Merchant Capital Trust II	Jul. 02, 2007 Subordinated Debt First Merchant Capital Trust II	Aug. 22, 2012 Junior Subordinated Debt CNBC Statutory Trust I	Feb. 22, 2011 Junior Subordinated Debt CNBC Statutory Trust I	Mar. 02, 2003 Junior Subordinated Debt CNBC Statutory Trust I	Jun. 30, 2010 Junior Subordinated Debt First Merchant Capital Trust III	Dec. 31, 2011 Senior Notes	Mar. 31, 2009 Senior Notes	Dec. 31, 2011 Revolving Credit Facility Line of Credit Bank Of America, N.A.	Dec. 31, 2012 Revolving Credit Facility Line of Credit Bank Of America, N.A.	Dec. 31, 2012 Term Loan Line of Credit Bank Of America, N.A.	Dec. 31, 2011 Term Loan Line of Credit Bank Of America, N.A.	Dec. 31, 2012 Subordinated Debt Line of Credit Bank Of America, N.A.	Dec. 31, 2011 Subordinated Debt Line of Credit Bank Of America, N.A.	Jun. 30, 2010 United States Department of Treasury Series A Preferred Stock	Jun. 30, 2010 United States Department of Treasury Trust Preferred Securities First Merchant Capital Trust III	Jun. 30, 2010 Parent Company Common Stock First Merchant Capital Trust III
Debt Instrument [Line Items]
Total Debt				$ 112,161,000	$ 194,974,000
Debt face amount								56,702,000	4,124,000		4,124,000	47,835,000		79,000,000
Interest rate on senior notes							6.495%				10.20%			2.625%
Description of variable rate basis						three-month LIBOR
Basis spread on variable rate						1.56%
Line of credit maximum borrowing capacity															55,000,000	55,000,000	5,000,000	5,000,000	50,000,000	50,000,000
Amount due in the event of a default																	5,000,000	5,000,000
The default rate percentage (per annum) on debt instrument																	3.00%	3.00%
The redemption price of the debt, percentage									104.59%	105.10%
Number of shares exchanged related to the Exchange Agreement with the U.S Treasury																					46,400
Private Stock Issuance (shares)	2,822,000																					46,400	1,435
Proceeds received by the Trust in connection with the issuance of common securities to the Corporation		$ 21,165,000	$ 24,150,000																				$ 1,400,000
The minimum return on average total assets covenent, percentage															0.75%
Effective rate on debt													3.812%