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Other Borrowings and Subordinated Notes and Debentures - Narrative (Details) - USD ($)	1 Months Ended		9 Months Ended	30 Months Ended
	Sep. 30, 2025	Mar. 31, 2018	Sep. 30, 2025	Sep. 30, 2025	Dec. 31, 2024	Apr. 30, 2022
Debt Instrument [Line Items]
Advances from FHLB	$ 2,700,000		$ 2,700,000	$ 2,700,000	$ 727,900,000
Mortgage loans and investment securities securing FHLB advances	6,640,000,000		6,640,000,000	6,640,000,000
Additional advances from FHLB	6,130,000,000		6,130,000,000	6,130,000,000
Subordinated Debt
Debt Instrument [Line Items]
Face amount of debt	$ 325,000,000.0	$ 330,000,000.0	$ 325,000,000.0	$ 325,000,000.0
Debt instrument, interest rate	6.25%	5.00%	6.25%	6.25%
Offering price, percent	100.00%	100.00%	100.00%	100.00%
Debt issuance costs incurred	$ 3,900,000	$ 3,600,000
Debt issuance costs, net	$ 565,000		$ 565,000	$ 565,000
Subordinated Debt \| Subordinated Notes Payable Due
Debt Instrument [Line Items]
Debt instrument, interest rate	6.00%		6.00%	6.00%
Long-term debt, gross	$ 0		$ 0	$ 0	$ 37,057,000
Subordinated Debt \| Spirit of Texas Bancshares, Inc. \| Subordinated Notes Payable Due
Debt Instrument [Line Items]
Debt instrument, interest rate	6.00%		6.00%	6.00%
Long-term debt, gross						$ 37,400,000
Subordinated Debt \| LIBOR
Debt Instrument [Line Items]
Floating rate (as percent)				2.15%
Subordinated Debt \| SOFR
Debt Instrument [Line Items]
Floating rate (as percent)	3.02%			0.26161%
Subordinated Debt \| SOFR \| Subordinated Notes Payable Due
Debt Instrument [Line Items]
Floating rate (as percent)			5.92%