Business Combinations - Narrative (Details) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		9 Months Ended			0 Months Ended	9 Months Ended			9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Nov. 12, 2013 Merger with CFS Bancorp, Inc. (Citizens)	Sep. 30, 2014 Merger with CFS Bancorp, Inc. (Citizens)	Nov. 12, 2013 Merger with CFS Bancorp, Inc. (Citizens) bank	Nov. 12, 2013 Merger with CFS Bancorp, Inc. (Citizens) CFS Bancorp	Sep. 30, 2014 Merger with Community Bancshares, Inc.	Jul. 21, 2014 Merger with Community Bancshares, Inc.	Jul. 21, 2014 Merger with Community Bancshares, Inc. Community Bancshares, Inc	Jul. 21, 2014 Merger with Community Bancshares, Inc. First Merchant Corp
Business Acquisition [Line Items]
Percentage of Interest Acquired								100.00%
Number of full service banking centers owned								20
Percentage of common stock received per shares owned through acquisition									0.65%
Core deposit intangible						7.1
Value of stock issued as a part of acquisition						$ 135,600,000
Core deposit intangible								7,313,000
Acquired intangible asset, expected useful life						10 years
Acquisition integration expenses							1,600,000
Salary and employee benefits expense	24,173,000	20,616,000	72,904,000	61,943,000			521,000
Equipment expenses	2,187,000	1,852,000	7,022,000	5,368,000			619,000
Marketing expenses	1,070,000	559,000	2,628,000	1,561,000			125,000
Assets	5,591,383,000		5,591,383,000		5,437,262,000						5,900,000,000	259,000,000	5,600,000,000
One-time expenses related to acquisition										$ 256,000