Loans and Allowance for Probable Loan Losses - Nonaccrual and Accruing Loans Past Due More Than 90 Days by Class (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 20,321	[1]	$ 4,096	[1]
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	631	[1]	716	[1]
1-4 Family Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	1,982	[1]	2,017	[1]
Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	2,096	[1]	675	[1]
Commercial Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	14,717	[1]	416	[1]
Municipal Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	249	[1]	0	[1]
Loans to Individuals
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 646	[1]	$ 272	[1]

[1] Excludes PCI loans measured at fair value at acquisition.