Securities (Securities Moody's and Standard & Poor's Bond Rating) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	$ 3,619,738	$ 2,639,241
States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	2,367,807	2,111,619
States and Political Subdivisions [Member] | Not Guaranteed or Pre-Refunded [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	804,737
States and Political Subdivisions [Member] | Guaranteed by the Texas PSF [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	1,541,913
States and Political Subdivisions [Member] | Guaranteed by Third Party
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	19,756
States and Political Subdivisions [Member] | Pre-Refunded [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	1,401
Other [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	1,500	$ 1,500
Aaa/AAA [Member] | States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	1,858,686
Aaa/AAA [Member] | States and Political Subdivisions [Member] | Not Guaranteed or Pre-Refunded [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	301,721
Aaa/AAA [Member] | States and Political Subdivisions [Member] | Guaranteed by the Texas PSF [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	1,541,913
Aaa/AAA [Member] | States and Political Subdivisions [Member] | Guaranteed by Third Party
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	13,651
Aaa/AAA [Member] | States and Political Subdivisions [Member] | Pre-Refunded [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	1,401
Aaa/AAA [Member] | Other [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	0
Aa/AA [Member] | States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	509,121
Aa/AA [Member] | States and Political Subdivisions [Member] | Not Guaranteed or Pre-Refunded [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	503,016
Aa/AA [Member] | States and Political Subdivisions [Member] | Guaranteed by the Texas PSF [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	0
Aa/AA [Member] | States and Political Subdivisions [Member] | Guaranteed by Third Party
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	6,105
Aa/AA [Member] | States and Political Subdivisions [Member] | Pre-Refunded [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	0
Aa/AA [Member] | Other [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	0
Not rated [Member] | States and Political Subdivisions [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	0
Not rated [Member] | States and Political Subdivisions [Member] | Not Guaranteed or Pre-Refunded [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	0
Not rated [Member] | States and Political Subdivisions [Member] | Guaranteed by the Texas PSF [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	0
Not rated [Member] | States and Political Subdivisions [Member] | Guaranteed by Third Party
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	0
Not rated [Member] | States and Political Subdivisions [Member] | Pre-Refunded [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	0
Not rated [Member] | Other [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,500