Loans and Allowance for Probable Loan Losses - Nonaccrual by Class of Loans (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Nonaccrual loans	[1],[2]	$ 17,148	$ 35,770
Construction Real Estate Loans
Financing Receivable, Past Due [Line Items]
Nonaccrual loans		412	12
1-4 Family Residential Real Estate Loans
Financing Receivable, Past Due [Line Items]
Nonaccrual loans		2,500	2,202
Commercial Real Estate Loans
Financing Receivable, Past Due [Line Items]
Nonaccrual loans		13,237	32,599
Commercial loans
Financing Receivable, Past Due [Line Items]
Nonaccrual loans		747	639
Loans to individuals
Financing Receivable, Past Due [Line Items]
Nonaccrual loans		$ 252	$ 318

[1] Includes $8.9 million and $10.9 million of restructured loans as of September 30, 2019 and December 31, 2018, respectively.

[2] Excludes 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.