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Securities Securities - Moody's and/or Standard & Poor's Bond Rating (Details) - USD ($)
$ in Thousands
Mar. 31, 2022
Dec. 31, 2021
Schedule of Held-to-maturity Securities [Line Items]    
Debt Securities, Held-to-maturity $ 1,665,623 $ 1,749,337
US States and Political Subdivisions Debt Securities [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Debt Securities, Held-to-maturity 1,137,154 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 1,029,256 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 107,898 104,295
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 198,932 196,674
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 91,034 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 107,898 104,295
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 467,039 460,648
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 467,039 460,648
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 471,183 563,251
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 471,183 563,251
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