Loans and Allowance for Probable Loan Losses - Allowance for Loan Losses Activity by Portfolio Segment (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Allowances for Loan Losses [Roll Forward]
Balance at beginning of period		$ 24,705	$ 25,072	$ 27,019	$ 20,781	[1]
Provision (reversal) for loan losses	[2]	1,005	975	2,593	5,991
Loans charged off		(1,000)	(662)	(5,682)	(2,346)
Recoveries of loans charged off		419	707	1,199	1,666
Balance at end of period		25,129	26,092	25,129	26,092
Provision (reversal) for PCI Loans		2	466	(161)	824
Construction Real Estate Loans
Allowances for Loan Losses [Roll Forward]
Balance at beginning of period		3,599	3,841	3,597	3,676	[1]
Provision (reversal) for loan losses		(35)	(717)	(33)	(538)
Loans charged off		0	0	0	(14)
Recoveries of loans charged off		12	0	12	0
Balance at end of period		3,576	3,124	3,576	3,124
1-4 Family Residential Real Estate Loans
Allowances for Loan Losses [Roll Forward]
Balance at beginning of period		3,522	2,730	3,844	2,445	[1]
Provision (reversal) for loan losses		18	585	(292)	906
Loans charged off		(24)	(8)	(42)	(65)
Recoveries of loans charged off		40	331	46	352
Balance at end of period		3,556	3,638	3,556	3,638
Commercial Real Estate Loans
Allowances for Loan Losses [Roll Forward]
Balance at beginning of period		10,534	14,036	13,968	10,821	[1]
Provision (reversal) for loan losses		27	290	(569)	3,499
Loans charged off		0	0	(2,876)	0
Recoveries of loans charged off		21	5	59	11
Balance at end of period		10,582	14,331	10,582	14,331
Commercial loans
Allowances for Loan Losses [Roll Forward]
Balance at beginning of period		5,546	2,560	3,974	2,094	[1]
Provision (reversal) for loan losses		618	533	2,618	1,194
Loans charged off		(394)	(13)	(975)	(270)
Recoveries of loans charged off		82	105	235	167
Balance at end of period		5,852	3,185	5,852	3,185
Municipal loans
Allowances for Loan Losses [Roll Forward]
Balance at beginning of period		530	859	525	860	[1]
Provision (reversal) for loan losses		17	5	22	4
Loans charged off		0	0	0	0
Recoveries of loans charged off		0	0	0	0
Balance at end of period		547	864	547	864
Loans to individuals
Allowances for Loan Losses [Roll Forward]
Balance at beginning of period		974	1,046	1,111	885	[1]
Provision (reversal) for loan losses		360	279	847	926
Loans charged off		(582)	(641)	(1,789)	(1,997)
Recoveries of loans charged off		264	266	847	1,136
Balance at end of period		$ 1,016	$ 950	$ 1,016	$ 950

[1] Loans acquired with the Diboll acquisition were measured at fair value on November 30, 2017 with no carryover of allowance for loan losses.

[2] Of the $1.0 million provision for loan losses for the three months ended September 30, 2019, $2,000 related to provision expense on PCI loans. Of the $2.6 million provision for loan losses for the nine months ended September 30, 2019, $161,000 related to provision expense reversed on PCI loans. Of the $975,000 and $6.0 million recorded in provision for loan losses for the three and nine months ended September 30, 2018, $466,000 and $824,000 related to provision expense on PCI loans, respectively.