Loans and Allowance for Probable Loan Losses - Aging of Past Due Loans by Class of Loan (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total Past Due		$ 17,974	$ 34,314
Current	[1]	3,481,943	3,278,485
Total ending loan balance		3,499,917	3,312,799
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		4,592	21,966
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		11,489	10,276
Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		1,893	2,072
Construction Real Estate Loans
Financing Receivable, Past Due [Line Items]
Total Past Due		1,031	934
Current	[1]	620,009	506,798
Total ending loan balance		621,040	507,732
Construction Real Estate Loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		478	627
Construction Real Estate Loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		207	307
Construction Real Estate Loans | Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		346	0
1-4 Family Residential Real Estate Loans
Financing Receivable, Past Due [Line Items]
Total Past Due		4,076	10,034
Current	[1]	788,562	784,465
Total ending loan balance		792,638	794,499
1-4 Family Residential Real Estate Loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		1,638	7,441
1-4 Family Residential Real Estate Loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		1,429	1,258
1-4 Family Residential Real Estate Loans | Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		1,009	1,335
Commercial Real Estate Loans
Financing Receivable, Past Due [Line Items]
Total Past Due		9,461	18,318
Current	[1]	1,226,846	1,175,800
Total ending loan balance		1,236,307	1,194,118
Commercial Real Estate Loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		282	10,663
Commercial Real Estate Loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		9,179	7,655
Commercial Real Estate Loans | Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		0	0
Commercial loans
Financing Receivable, Past Due [Line Items]
Total Past Due		2,554	3,242
Current	[1]	379,523	353,407
Total ending loan balance		382,077	356,649
Commercial loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		1,496	1,946
Commercial loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		584	705
Commercial loans | Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		474	591
Municipal loans
Financing Receivable, Past Due [Line Items]
Total Past Due		0	0
Current	[1]	366,906	353,370
Total ending loan balance		366,906	353,370
Municipal loans | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		0	0
Municipal loans | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		0	0
Municipal loans | Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		0	0
Loans to individuals
Financing Receivable, Past Due [Line Items]
Total Past Due		852	1,786
Current	[1]	100,097	104,645
Total ending loan balance		100,949	106,431
Loans to individuals | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		698	1,289
Loans to individuals | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		90	351
Loans to individuals | Greater than 90 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due		$ 64	$ 146

[1] Includes PCI loans measured at fair value at acquisition if the timing and amount of cash flows expected to be collected from those sales can be reasonably estimated.