Securities Securities - Moody's and/or Standard & Poor's Bond Rating (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	$ 3,511,466	$ 3,534,085
US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	2,329,853	2,338,745
Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,500	1,500
Moody's, Aaa Rating \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,829,622	1,834,432
Moody's, Aaa Rating \| Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Moody's, Aa1 Rating \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	500,231	504,313
Moody's, Aa1 Rating \| Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Not rated [Member] \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Not rated [Member] \| Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,500	1,500
Not Guaranteed or Pre-Refunded [Member] \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	794,868	799,508
Not Guaranteed or Pre-Refunded [Member] \| Moody's, Aaa Rating \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	300,754	301,310
Not Guaranteed or Pre-Refunded [Member] \| Moody's, Aa1 Rating \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	494,114	498,198
Not Guaranteed or Pre-Refunded [Member] \| Not rated [Member] \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Guaranteed by the Texas PSF [Member] \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,496,775	1,504,951
Guaranteed by the Texas PSF [Member] \| Moody's, Aaa Rating \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,496,775	1,504,951
Guaranteed by the Texas PSF [Member] \| Moody's, Aa1 Rating \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Guaranteed by the Texas PSF [Member] \| Not rated [Member] \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Guaranteed by Third Party \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	19,754	19,755
Guaranteed by Third Party \| Moody's, Aaa Rating \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	13,637	13,640
Guaranteed by Third Party \| Moody's, Aa1 Rating \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	6,117	6,115
Guaranteed by Third Party \| Not rated [Member] \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Pre-Refunded [Member] \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	18,456	14,531
Pre-Refunded [Member] \| Moody's, Aaa Rating \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	18,456	14,531
Pre-Refunded [Member] \| Moody's, Aa1 Rating \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	0
Pre-Refunded [Member] \| Not rated [Member] \| US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	$ 0	$ 0