Loans and Allowance for Probable Loan Losses - Interest Income on Impaired Loans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Impaired Financing Receivable, Average Recorded Investment and Interest Income on Impaired Loans [Abstract]
Average recorded investment	$ 14,099	[1]	$ 10,897
Interest income recognized	71	[1]	60
Construction
Impaired Financing Receivable, Average Recorded Investment and Interest Income on Impaired Loans [Abstract]
Average recorded investment	2,401	[1]	1,371
Interest income recognized	23	[1]	0
1-4 Family Residential
Impaired Financing Receivable, Average Recorded Investment and Interest Income on Impaired Loans [Abstract]
Average recorded investment	4,000	[1]	2,471
Interest income recognized	17	[1]	12
Other
Impaired Financing Receivable, Average Recorded Investment and Interest Income on Impaired Loans [Abstract]
Average recorded investment	1,945	[1]	1,717
Interest income recognized	13	[1]	12
Commercial Loans
Impaired Financing Receivable, Average Recorded Investment and Interest Income on Impaired Loans [Abstract]
Average recorded investment	4,530	[1]	1,480
Interest income recognized	8	[1]	4
Municipal Loans
Impaired Financing Receivable, Average Recorded Investment and Interest Income on Impaired Loans [Abstract]
Average recorded investment	761	[1]	759
Interest income recognized	10	[1]	0
Loans to Individuals
Impaired Financing Receivable, Average Recorded Investment and Interest Income on Impaired Loans [Abstract]
Average recorded investment	462	[1]	3,099
Interest income recognized	$ 0	[1]	$ 32

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