Loans and Allowance for Probable Loan Losses - Allowance for Loan Losses, Loan Portfolio, by Impairment Method (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Financing Receivable, Individually Evaluated for Impairment	$ 8,882	$ 10,514
Financing Receivable, Collectively Evaluated for Impairment	1,284,547	1,252,463
Total ending loans balance	1,293,429	1,262,977
Construction
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Financing Receivable, Individually Evaluated for Impairment	1,581	2,465
Financing Receivable, Collectively Evaluated for Impairment	121,912	111,279
Total ending loans balance	123,493	113,744
1-4 Family residential
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Financing Receivable, Individually Evaluated for Impairment	3,062	2,799
Financing Receivable, Collectively Evaluated for Impairment	382,179	366,046
Total ending loans balance	385,241	368,845
Other
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Financing Receivable, Individually Evaluated for Impairment	1,879	2,613
Financing Receivable, Collectively Evaluated for Impairment	230,753	234,147
Total ending loans balance	232,632	236,760
Commercial loans
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Financing Receivable, Individually Evaluated for Impairment	1,834	2,043
Financing Receivable, Collectively Evaluated for Impairment	152,151	158,015
Total ending loans balance	153,985	160,058
Municipal loans
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Financing Receivable, Individually Evaluated for Impairment	0	0
Financing Receivable, Collectively Evaluated for Impairment	224,134	220,947
Total ending loans balance	224,134	220,947
Loans to individuals
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Financing Receivable, Individually Evaluated for Impairment	526	594
Financing Receivable, Collectively Evaluated for Impairment	173,418	162,029
Total ending loans balance	$ 173,944	$ 162,623