Loans and Allowance for Probable Loan Losses - Troubled Debt Restructurings (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015 USD ($) contract	Jun. 30, 2014 USD ($) contract	Jun. 30, 2015 USD ($) contract	Jun. 30, 2014 USD ($) contract
Troubled Debt Restructuring [Abstract]
Extend Amortization Period		$ 304	$ 60	$ 334	$ 313
Interest Rate Reductions		0	0	0	299
Combination	[1]	58	388	1,108	514
Total Modifications		$ 362	$ 448	$ 1,442	$ 1,126
Number of Loans | contract		8	3	14	12
1-4 Family Residential
Troubled Debt Restructuring [Abstract]
Extend Amortization Period		$ 0	$ 0	$ 0	$ 0
Interest Rate Reductions		0	0	0	284
Combination	[1]	0	0	262	0
Total Modifications		$ 0	$ 0	$ 262	$ 284
Number of Loans | contract		0	0	2	1
Other
Troubled Debt Restructuring [Abstract]
Extend Amortization Period		$ 0	$ 0	$ 30	$ 0
Interest Rate Reductions		0	0	0	0
Combination	[1]	0	388	0	413
Total Modifications		$ 0	$ 388	$ 30	$ 413
Number of Loans | contract		0	1	1	2
Commercial Loans
Troubled Debt Restructuring [Abstract]
Extend Amortization Period		$ 300	$ 60	$ 300	$ 313
Interest Rate Reductions		0	0	0	0
Combination	[1]	0	0	762	56
Total Modifications		$ 300	$ 60	$ 1,062	$ 369
Number of Loans | contract		1	2	2	5
Loans to Individuals
Troubled Debt Restructuring [Abstract]
Extend Amortization Period		$ 4	$ 0	$ 4	$ 0
Interest Rate Reductions		0	0	0	15
Combination	[1]	58	0	84	45
Total Modifications		$ 62	$ 0	$ 88	$ 60
Number of Loans | contract		7	0	9	4

[1] These modifications include an extension of the amortization period and interest rate reduction.