Investment Securities, HTM Credit Quality Indicators (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	$ 1,990,940	$ 2,205,083
US Treasury Securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	999	999
Obligations of state and political subdivisions (municipal securities)
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	1,444,325	$ 1,418,569
FNMA/FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	63,142
GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	159,455
us-Federal Home Loan Mortgage Corporation Certificates And Obligations (FHLMC) And Federal National Mortgage Association Certificates And Obligations (FNMA), Commercial
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	11,220
GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	311,798
AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	1,114,082
AAA | US Treasury Securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	999
AAA | Obligations of state and political subdivisions (municipal securities)
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	567,468
AAA | FNMA/FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	63,142
AAA | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	159,455
AAA | us-Federal Home Loan Mortgage Corporation Certificates And Obligations (FHLMC) And Federal National Mortgage Association Certificates And Obligations (FNMA), Commercial
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	11,220
AAA | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	311,798
AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	860,465
AA | US Treasury Securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	0
AA | Obligations of state and political subdivisions (municipal securities)
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	860,465
AA | FNMA/FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	0
AA | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	0
AA | us-Federal Home Loan Mortgage Corporation Certificates And Obligations (FHLMC) And Federal National Mortgage Association Certificates And Obligations (FNMA), Commercial
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	0
AA | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	0
A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	16,393
A | US Treasury Securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	0
A | Obligations of state and political subdivisions (municipal securities)
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	16,393
A | FNMA/FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	0
A | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	0
A | us-Federal Home Loan Mortgage Corporation Certificates And Obligations (FHLMC) And Federal National Mortgage Association Certificates And Obligations (FNMA), Commercial
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	0
A | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	$ 0