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Securities Securities - Moody's and/or Standard & Poor's Bond Rating (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | $ 3,601,039 | $ 3,619,738
US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 2,360,119 | 2,367,807
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 | 1,855,277 | 1,858,686
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 | 504,842 | 509,121
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 | 800,354 | 804,737
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 | 301,620 | 301,721
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 | 498,734 | 503,016
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 | 1,538,918 | 1,541,913
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 | 1,538,918 | 1,541,913
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 | 19,756
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 | 13,648 | 13,651
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 | 6,108 | 6,105
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 | 1,401
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 | 1,401
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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