Loans and Allowance for Probable Loan Losses - Loans by Portfolio Segment (Details) (USD $)	Mar. 31, 2015		Dec. 31, 2014
Loans and Leases Receivable Disclosure [Abstract]
Total ending loans balance	$ 2,174,614,000	[1]	$ 2,181,133,000	[1]
Less: Allowance for loan losses	16,926,000		13,292,000
Net loans	2,157,688,000		2,167,841,000
Construction
Loans and Leases Receivable Disclosure [Abstract]
Total ending loans balance	237,236,000		243,486,000
1-4 Family Residential
Loans and Leases Receivable Disclosure [Abstract]
Total ending loans balance	691,955,000		689,288,000
Other
Loans and Leases Receivable Disclosure [Abstract]
Total ending loans balance	502,476,000		485,226,000
Commercial Loans
Loans and Leases Receivable Disclosure [Abstract]
Total ending loans balance	249,407,000		235,356,000
Municipal Loans
Loans and Leases Receivable Disclosure [Abstract]
Total ending loans balance	252,756,000		257,492,000
Loans to Individuals
Loans and Leases Receivable Disclosure [Abstract]
Total ending loans balance	240,784,000		270,285,000
OmniAmerican Bancorp, Inc.
Loans and Financing Receivable [Line Items]
Loans acquired carrying amount	$ 727,700,000		$ 763,300,000

[1] Includes approximately $727.7 million and $763.3 million of loans acquired with the Omni acquisition as of March 31, 2015 and December 31, 2014, respectively. These loans were measured at fair value at the acquisition date with no carryover of allowance for loan loss.