Acquisitions - Acquisitions (Details) $ in Thousands		6 Months Ended
	Dec. 17, 2014 banking_office	Jun. 30, 2015 USD ($)
Goodwill [Roll Forward]
Balance as of December 31, 2014		$ 91,372
Less: measurement period adjustments		(801)
Balance as of June 30, 2015		90,571
OmniAmerican Bancorp, Inc.
Business Acquisition [Line Items]
Date of acquisition	Dec. 17, 2014
Percentage of voting interests acquired	100.00%
Name of acquired entity	OmniAmerican Bancorp, Inc.
Number of banking offices | banking_office	14
OmniAmerican Bancorp, Inc. | Other investments
Business Acquisition [Line Items]
Adjustment to the fair value of Visa Class B stock included in other investments		$ 1,400