Loans and Allowance for Probable Loan Losses - Loans by Credit Quality Indicator (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	$ 1,370,393	$ 1,351,273
Construction
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	135,237	125,219
1-4 Family Residential
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	395,809	390,499
Other
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	272,868	262,536
Commercial Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	154,524	157,655
Municipal Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	240,114	245,550
Loans to Individuals
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	171,841	169,814
Pass
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	1,331,889	1,315,354
Pass | Construction
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	129,202	121,280
Pass | 1-4 Family Residential
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	385,940	380,741
Pass | Other
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	259,133	249,381
Pass | Commercial Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	147,585	150,683
Pass | Municipal Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	239,105	244,505
Pass | Loans to Individuals
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	170,924	168,764
Pass Watch
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	6,662	5,042
Pass Watch | Construction
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	1,746	0
Pass Watch | 1-4 Family Residential
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	1,487	1,626
Pass Watch | Other
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	2,570	2,553
Pass Watch | Commercial Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	824	836
Pass Watch | Municipal Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	0	0
Pass Watch | Loans to Individuals
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	35	27
Special Mention
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	6,638	9,153
Special Mention | Construction
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	2,247	1,419
Special Mention | 1-4 Family Residential
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	1,747	3,025
Special Mention | Other
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	2,644	4,698
Special Mention | Commercial Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	0	9
Special Mention | Municipal Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	0	0
Special Mention | Loans to Individuals
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	0	2
Substandard
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	24,104	20,832
Substandard | Construction
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	2,019	2,454
Substandard | 1-4 Family Residential
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	6,056	4,901
Substandard | Other
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	8,509	5,887
Substandard | Commercial Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	5,850	5,826
Substandard | Municipal Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	1,009	1,045
Substandard | Loans to Individuals
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	661	719
Doubtful
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	1,100	892
Doubtful | Construction
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	23	66
Doubtful | 1-4 Family Residential
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	579	206
Doubtful | Other
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	12	17
Doubtful | Commercial Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	265	301
Doubtful | Municipal Loans
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	0	0
Doubtful | Loans to Individuals
Financing Receivable, Recorded Investment, Credit Quality Indicator [Abstract]
Total ending loans balance	$ 221	$ 302