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Securities (Securities Moody's and Standard & Poor's Bond Rating) (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to Maturity, Amortized Cost | $ 1,945,833 | $ 2,030,005
States and Political Subdivisions [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to Maturity, Amortized Cost | 1,415,549 | 1,497,644
States and Political Subdivisions [Member] | Not Guaranteed or Pre-Refunded [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to Maturity, Amortized Cost | 284,805 |
States and Political Subdivisions [Member] | Guaranteed by the Texas PSF [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to Maturity, Amortized Cost | 659,999 |
States and Political Subdivisions [Member] | Pre-Refunded [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to Maturity, Amortized Cost | 470,745 |
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 | 1,245,760 |
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 | 115,016 |
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 | 659,999 |
Aaa/AAA [Member] | States and Political Subdivisions [Member] | Pre-Refunded [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to Maturity, Amortized Cost | 470,745 |
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 | 107,065 |
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 | 107,065 |
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] | 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 |
A [Member] | States and Political Subdivisions [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to Maturity, Amortized Cost | 62,724 |
A [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 | 62,724 |
A [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 |
A [Member] | States and Political Subdivisions [Member] | Pre-Refunded [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Held to Maturity, Amortized Cost | 0 |
A [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] | 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 |

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