Loans and Allowance for Probable Loan Losses - Allowance for Loan Losses, Loan Portfolio, by Impairment Method (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	$ 25,477	$ 36,247
Loans collectively evaluated for impairment	3,435,001	3,230,150
Purchased credit impaired loans	39,439	46,402
Total ending loan balance	3,499,917	3,312,799
Construction Real Estate Loans
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	412	12
Loans collectively evaluated for impairment	620,497	507,564
Purchased credit impaired loans	131	156
Total ending loan balance	621,040	507,732
1-4 Family Residential Real Estate Loans
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	1,231	1,215
Loans collectively evaluated for impairment	783,504	782,614
Purchased credit impaired loans	7,903	10,670
Total ending loan balance	792,638	794,499
Commercial Real Estate Loans
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	21,180	33,013
Loans collectively evaluated for impairment	1,185,138	1,128,220
Purchased credit impaired loans	29,989	32,885
Total ending loan balance	1,236,307	1,194,118
Commercial loans
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	2,182	1,394
Loans collectively evaluated for impairment	378,635	353,036
Purchased credit impaired loans	1,260	2,219
Total ending loan balance	382,077	356,649
Municipal loans
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	352	429
Loans collectively evaluated for impairment	366,554	352,941
Purchased credit impaired loans	0	0
Total ending loan balance	366,906	353,370
Loans to individuals
Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Line Items]
Loans individually evaluated for impairment	120	184
Loans collectively evaluated for impairment	100,673	105,775
Purchased credit impaired loans	156	472
Total ending loan balance	$ 100,949	$ 106,431

[Line Items] for Schedule of Allowance For Loan Losses, Loan Balance, by Impairment Method [Table]

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- Definition

The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses). This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=118939070&loc=d3e15032-111544

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-Publisher FASB
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-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118938799&loc=d3e13503-111538

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- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

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The balance of financing receivables that were individually evaluated for impairment.

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- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

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