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Securities Securities - Moody's and/or Standard & Poor's Bond Rating (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | $ 1,951,544 | $ 2,030,005
US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 1,420,754 | 1,497,644
Other Investments [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 1,500 | $ 1,500
Aaa/AAA [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 1,245,284 |
Aaa/AAA [Member] | Other Investments [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 0 |
Aa/AA [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 112,614 |
Aa/AA [Member] | Other Investments [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 0 |
A [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 62,856 |
A [Member] | Other Investments [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 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 |
Not rated [Member] | Other Investments [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 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 | 290,855 |
Not Guaranteed or Pre-Refunded [Member] | Aaa/AAA [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 115,385 |
Not Guaranteed or Pre-Refunded [Member] | Aa/AA [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 112,614 |
Not Guaranteed or Pre-Refunded [Member] | A [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 62,856 |
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 |
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 | 797,659 |
Guaranteed by the Texas PSF [Member] | Aaa/AAA [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 797,659 |
Guaranteed by the Texas PSF [Member] | Aa/AA [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 0 |
Guaranteed by the Texas PSF [Member] | A [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 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 |
Pre-Refunded [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 332,240 |
Pre-Refunded [Member] | Aaa/AAA [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 332,240 |
Pre-Refunded [Member] | Aa/AA [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 0 |
Pre-Refunded [Member] | A [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Debt Securities, Held-to-maturity | 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 |

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