Other Borrowings and Subordinated Notes and Debentures - Narrative (Details) - USD ($)	6 Months Ended	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2023	Dec. 31, 2022	Mar. 31, 2018
Debt Instrument [Line Items]
Advances from Federal Home Loan Banks	$ 1,330,000,000	$ 1,330,000,000	$ 838,500,000
Mortgage loans and investment securities securing FHLB advances	7,050,000,000.00	7,050,000,000.00
Additional advances from Federal Home Loan Bank	$ 5,370,000,000	$ 5,370,000,000
Subordinated Debt
Debt Instrument [Line Items]
Face amount of debt				$ 330,000,000
Debt instrument, interest rate				5.00%
Offering price, percent				100.00%
Debt issuance costs				$ 3,600,000
Subordinated Debt | LIBOR
Debt Instrument [Line Items]
Floating rate (as percent)	2.15%
Subordinated Debt | SOFR
Debt Instrument [Line Items]
Floating rate (as percent)	0.26161%	0.26161%
Subordinated Debt | Subordinated notes payable, net of premium adjustments, due 7/31/2030, fixed-to-floating rate (fixed rate of 6.00% through 7/30/2025, floating rate of 5.92% above the three month SOFR rate, reset quarterly)
Debt Instrument [Line Items]
Debt instrument, interest rate	6.00%	6.00%
Long-term debt, gross	$ 37,199,000	$ 37,199,000	$ 37,285,000
Subordinated Debt | Subordinated notes payable, net of premium adjustments, due 7/31/2030, fixed-to-floating rate (fixed rate of 6.00% through 7/30/2025, floating rate of 5.92% above the three month SOFR rate, reset quarterly) | Spirit of Texas Bancshares, Inc.
Debt Instrument [Line Items]
Debt instrument, interest rate	6.00%	6.00%
Long-term debt, gross	$ 37,400,000	$ 37,400,000
Subordinated Debt | Subordinated notes payable, net of premium adjustments, due 7/31/2030, fixed-to-floating rate (fixed rate of 6.00% through 7/30/2025, floating rate of 5.92% above the three month SOFR rate, reset quarterly) | SOFR
Debt Instrument [Line Items]
Floating rate (as percent)		5.92%
Subordinated Debt | Subordinated notes payable, net of premium adjustments, due 7/31/2030, fixed-to-floating rate (fixed rate of 6.00% through 7/30/2025, floating rate of 5.92% above the three month SOFR rate, reset quarterly) | SOFR | Spirit of Texas Bancshares, Inc.
Debt Instrument [Line Items]
Floating rate (as percent)		5.92%