Loans and Allowance for Probable Loan Losses - Interest Income on Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Financing Receivable, Impaired [Line Items]
Average recorded investment	$ 34,730	$ 40,061	$ 39,110	$ 31,034
Interest income recognized	348	96	1,098	252
Construction Real Estate Loans
Financing Receivable, Impaired [Line Items]
Average recorded investment	490	252	304	150
Interest income recognized	3	3	12	4
1-4 Family Residential Real Estate Loans
Financing Receivable, Impaired [Line Items]
Average recorded investment	7,322	3,976	6,569	3,951
Interest income recognized	124	49	304	138
Commercial Real Estate Loans
Financing Receivable, Impaired [Line Items]
Average recorded investment	23,018	32,580	28,549	24,149
Interest income recognized	166	16	589	36
Commercial loans
Financing Receivable, Impaired [Line Items]
Average recorded investment	3,195	2,543	2,822	2,073
Interest income recognized	45	21	162	51
Municipal loans
Financing Receivable, Impaired [Line Items]
Average recorded investment	391	466	414	488
Interest income recognized	6	6	18	20
Loans to individuals
Financing Receivable, Impaired [Line Items]
Average recorded investment	314	244	452	223
Interest income recognized	$ 4	$ 1	$ 13	$ 3

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- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5258-111524

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- Definition

Amount of interest income recognized on a cash-basis method of accounting that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

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