Loans and Allowance for Credit Losses - Narrative (Details)		12 Months Ended
	Apr. 08, 2022 USD ($)	Dec. 31, 2023 USD ($) loan payment	Dec. 31, 2022 USD ($) loan	Dec. 31, 2021 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		$ 16,845,670,000	$ 16,142,124,000
Deferred origination fees, net		6,700,000	26,400,000
Financing receivable, accrued interest excluded from amortized costs		77,100,000	65,400,000
Nonaccrual loans with no allowance for credit loss		$ 3,200,000	$ 16,900,000
Number of loan modifications | loan			4
Number of loan modifications with payment default | loan		0
Foreclosed assets and other real estate owned		$ 4,073,000	$ 2,887,000
Financing receivable, troubled debt restructuring, premodification			760,000
Financial impact on date of restructure			$ 0
Number of loans with subsequent default | loan			1
TDR loan, payment default			$ 7,800
Financing receivable, collateral dependent, amount		144,604,000	70,926,000
Unfunded commitments		(16,300,000)	19,453,000	$ 0
Provision for credit loss expense		(47,424,000)	5,379,000	31,209,000
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		$ 2,500,000	3,000,000
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		506,000	$ 853,000
Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loan modifications | loan			37
Provision for credit loss expense		(47,424,000)	$ 5,379,000	31,209,000
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		13,338,186,000	$ 12,581,262,000
Number of loan modifications | loan			4
Financing receivable, troubled debt restructuring, premodification			$ 760,000
Financial impact on date of restructure			0
Real Estate | Spirit of Texas Bancshares, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred origination fees, net		998,000
Real Estate | OREO Received in Full or Partial Satisfaction of Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, troubled debt restructuring, premodification			$ 0
Real Estate | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loan modifications | loan			36
Provision for credit loss expense		(36,381,000)	$ 34,456,000	7,918,000
Unfunded commitments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Off-balance sheet, credit loss, liability, credit loss expense (reversal)		16,300,000	16,000,000	0
Unfunded commitments, allowance for credit loss		25,600,000	41,900,000
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		$ 2,722,886,000	$ 2,837,913,000
Number of loan modifications | loan		1
Number of interest only loan payments | payment		2
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]
Number of loan modifications | loan			1
Provision for credit loss expense		(5,934,000)	$ (22,412,000)	$ 22,031,000
Paycheck Protection Program Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		4,800,000	8,900,000
Other commercial | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		7,552,410,000	7,468,498,000
Charge-off		9,600,000
Financing receivable, collateral dependent, amount		$ 76,229,000	$ 65,450,000