Acquisitions and Divestitures - Acquisitions Narrative (Details) $ / shares in Units, $ in Thousands, shares in Millions			6 Months Ended	12 Months Ended
	Apr. 17, 2015 USD ($) bank $ / shares	Nov. 07, 2014 USD ($) bank $ / shares shares	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Business Acquisition [Line Items]
Cash paid in acquisition			$ 14,500
Less: Common stock issued			$ 0
Merger with C Financial Corporation
Business Acquisition [Line Items]
Percentage of interest acquired	100.00%
Number of banking centers acquired | bank	6
Purchase price	$ 14,500
Cash received by shareholders for each share of common stock held (usd per share) | $ / shares	$ 6.738
Core deposit intangible	$ 981
Acquired intangible asset, expected useful life	10 years
Merger with Community Bancshares, Inc.
Business Acquisition [Line Items]
Percentage of interest acquired		100.00%
Number of banking centers acquired | bank		10
Purchase price		$ 49,189
Core deposit intangible		$ 4,658
Acquired intangible asset, expected useful life		10 years
Share price (usd per share) | $ / shares		$ 85.94
Cash paid in acquisition		$ 14,200
Stock issued as a part of acquisition (in shares) | shares		1.6
Less: Common stock issued		$ 35,000
Pro forma operating revenue				$ 1,600
Pro forma nonrecurring expense				$ 1,800
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
Pro forma nonrecurring expense					$ 9,500