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Securities Securities - Moody's and/or Standard & Poor's Bond Rating (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | $ 3,628,433 | $ 2,639,241
US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 2,368,778 | 2,111,619
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 | 1,859,485 | 1,817,604
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 | 509,293 | 294,015
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 | 805,012 | 567,216
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 | 301,822 | 273,201
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 | 503,190 | 294,015
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 | 1,542,919 | 1,422,442
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 | 1,542,919 | 1,422,442
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 | 19,756 | 0
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 | 13,653 | 0
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 | 6,103 | 0
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 | 1,091 | 121,961
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 | 1,091 | 121,961
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

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