Loans and Allowance for Credit Losses - Narrative (Details)	12 Months Ended
	Dec. 31, 2024 USD ($) loan	Dec. 31, 2023 USD ($) loan	Dec. 31, 2022 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 17,005,937,000	$ 16,845,670,000
Deferred origination fees, net	7,200,000	13,200,000
Deferred origination costs	9,600,000	6,500,000
Financing receivable, accrued interest excluded from amortized costs	78,800,000	77,100,000
Nonaccrual loans with no allowance for credit loss	$ 1,700,000	$ 3,200,000
Number of loan modifications | loan	1	1
Extension of term of loan	1 year 6 months
Charge-off		$ 18,800
Number of loan modifications with payment default | loan		0
Financing receivable, collateral dependent, amount	$ 102,580,000	$ 144,604,000
Provision for credit loss expense	(46,785,000)	(47,424,000)	$ 5,379,000
Spirit of Texas Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred origination fees, net	$ 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	$ 4,000,000.0	2,500,000
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	1,300,000	506,000
Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit loss expense	(46,785,000)	(47,424,000)	5,379,000
Loans, Excluding Acquired Loans | Restaurant, hospitality, student housing and office space
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit loss expense			16,000,000
Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	13,391,531,000	13,338,186,000
Real Estate | Spirit of Texas Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred origination fees, net	998,000
Real Estate | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit loss expense	(10,249,000)	(36,381,000)	34,456,000
Unfunded commitments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Off-balance sheet, credit loss, liability, credit loss expense (reversal)	0	16,300,000	16,000,000
Unfunded commitments, allowance for credit loss	25,600,000	25,600,000
Unfunded commitments | Spirit of Texas Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Off-balance sheet, credit loss, liability, credit loss expense (reversal)			3,500,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,695,329,000	$ 2,722,886,000
Number of interest only loan payments		2 months
Commercial | Spirit of Texas Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred origination fees, net	378,000
Commercial | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision for credit loss expense	(30,389,000)	$ (5,934,000)	$ (22,412,000)
Paycheck Protection Program Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,600,000	4,800,000
Other commercial | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	7,912,336,000	7,552,410,000
Charge-off		9,600,000
Financing receivable, collateral dependent, amount	$ 69,429,000	$ 76,229,000