Business Combinations - Narrative (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	0 Months Ended	12 Months Ended		0 Months Ended
	Nov. 07, 2014 bank_branch	Dec. 31, 2014	Dec. 31, 2013	Apr. 17, 2015	Mar. 31, 2015
Business Acquisition [Line Items]
Assets		$ 5,824,127,000			$ 5,877,521,000
Merger with Community Bancshares, Inc.
Business Acquisition [Line Items]
Percentage of interest acquired	100.00%
Number of banking centers acquired	10
Share price	$ 85.94
Cash paid in acquisition	14,200,000
Stock issued as a part of acquisition (in shares)	1.6
Value of stock issued as a part of acquisition	35,000,000
Purchase price	49,189,000
Core deposit intangible	4,658,000
Acquired intangible asset, expected useful life	10 years
Pro forma operating revenue		1,600,000
Pro forma nonrecurring expense		1,800,000
Merger with Community Bancshares, Inc. | Common Stock
Business Acquisition [Line Items]
Number of shares of common stock	4.0926
Merger with CFS Bancorp, Inc. (Citizens)
Business Acquisition [Line Items]
Pro forma operating revenue			4,900,000
Pro forma nonrecurring expense			9,500,000
Merger with C Financial Corporation | Subsequent Event
Business Acquisition [Line Items]
Total purchase price				14,500,000
Assets				6,000,000,000
Merger with C Financial Corporation | First Merchants Corporation | Subsequent Event
Business Acquisition [Line Items]
Assets				5,800,000,000
Merger with C Financial Corporation | C Financial Corporation | Subsequent Event
Business Acquisition [Line Items]
Cash received by shareholders for each share of common stock held				$ 6.738
Assets				$ 138,000,000