Loans and Allowance for Credit Losses - Collateral Dependent Loans (Details) $ in Thousands	Sep. 30, 2020 USD ($)
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	$ 71,727
Real Estate Collateral
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	48,618
Energy
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	19,100
Other Collateral
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	4,009
Real Estate | Construction and development
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	1,538
Real Estate | Construction and development | Real Estate Collateral
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	1,538
Real Estate | Construction and development | Energy
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	0
Real Estate | Construction and development | Other Collateral
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	0
Real Estate | Single family residential
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	6,722
Real Estate | Single family residential | Real Estate Collateral
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	6,722
Real Estate | Single family residential | Energy
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	0
Real Estate | Single family residential | Other Collateral
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	0
Real Estate | Other commercial
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	40,358
Real Estate | Other commercial | Real Estate Collateral
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	40,358
Real Estate | Other commercial | Energy
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	0
Real Estate | Other commercial | Other Collateral
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	0
Commercial | Commercial
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	23,109
Commercial | Commercial | Real Estate Collateral
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	0
Commercial | Commercial | Energy
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	19,100
Commercial | Commercial | Other Collateral
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, collateral dependent, amount	$ 4,009