Securities Securities - Moody's and/or Standard & Poor's Bond Rating (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	$ 1,969,126	$ 2,030,005
US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,437,758	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	1,263,344
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	111,427
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	62,987
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	305,718
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	131,304
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	111,427
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	62,987
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	840,004
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	840,004
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	292,036
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	292,036
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