Loans and Allowance for Credit Losses - Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2022 USD ($)	Dec. 31, 2022 USD ($) loan	Dec. 31, 2021 USD ($) loan	Dec. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 16,142,124,000	$ 16,142,124,000	$ 12,012,503,000
Non-credit related discount		26,400,000	21,500,000
Financing receivable, accrued interest excluded from amortized costs	65,400,000	65,400,000	39,800,000
Nonaccrual loans with no allowance for credit loss	16,900,000	$ 16,900,000	$ 14,500,000
Number of loan modifications | loan		4	4
Balance Prior to TDR		$ 760,000	$ 1,058,000
Allowance for credit losses on loans	196,955,000	$ 196,955,000	$ 205,332,000
Number of previously restructured loans paid off | loan		15	9
Restructured loans paid off		$ 3,169,776	$ 1,002,874
Financial impact on date of restructure			5,129
Number of loans with subsequent default | loan		1
TDR loan, payment default		$ 7,800
Foreclosed assets and other real estate owned	2,887,000	2,887,000	6,032,000
Financing receivable, collateral dependent, amount	70,926,000	70,926,000	47,089,000
Unfunded commitments, allowance for credit loss	41,900,000	41,900,000	22,400,000
Off-balance sheet, credit loss, liability, credit loss expense (reversal)	16,000,000	3,500,000		$ 0
Unfunded commitments	16,000,000	19,453,000	0	$ (10,000,000)
Spirit of Texas Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-credit related discount		1,377,000
Off-balance sheet, credit loss, liability, credit loss expense (reversal)		$ 30,300,000
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, term of loan		1 year
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, term of loan		3 years
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in process of foreclosure, amount	3,009,000	$ 3,009,000	1,806,000
OREO Received in Full or Partial Satisfaction of Loans | Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Foreclosed assets and other real estate owned	853,000	853,000	831,000
Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	12,581,262,000	$ 12,581,262,000	$ 9,167,250,000
Number of loan modifications | loan		4	4
Balance Prior to TDR		$ 760,000	$ 1,058,000
Financial impact on date of restructure		0	0
Real Estate | Spirit of Texas Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-credit related discount		998,000
Real Estate | OREO Received in Full or Partial Satisfaction of Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance Prior to TDR		0	0
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,837,913,000	2,837,913,000	2,160,760,000
Commercial | Spirit of Texas Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-credit related discount		378,000
Single family residential | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,546,115,000	$ 2,546,115,000	$ 2,101,975,000
Number of loan modifications | loan		4	3
Balance Prior to TDR		$ 760,000	$ 274,000
Financial impact on date of restructure		0	0
Financing receivable, collateral dependent, amount	0	0	1,838,000
Other commercial | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	7,468,498,000	$ 7,468,498,000	$ 5,738,904,000
Number of loan modifications | loan		0	1
Balance Prior to TDR		$ 0	$ 784,000
Financial impact on date of restructure		0	0
Financing receivable, collateral dependent, amount	65,450,000	65,450,000	32,849,000
Paycheck Protection Program Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 8,900,000	$ 8,900,000	$ 116,700,000