Securities Securities - Moody's and/or Standard & Poor's Bond Rating (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	$ 2,174,242	$ 1,749,337
US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,646,345	1,220,573
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	1,457,704	1,116,278
Aaa/AAA [Member] | Other Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0	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	188,641	104,295
Aa/AA [Member] | 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	408,821	196,674
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	220,180	92,379
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	188,641	104,295
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	1,116,482	460,648
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	1,116,482	460,648
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	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
Pre-Refunded [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	121,042	563,251
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	121,042	563,251
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	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