Investment Securities, HTM Credit Quality Indicators (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	$ 2,149,434	$ 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	1,426,543	$ 1,418,569
FNMA/FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	78,125
GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	245,675
GNMA commercial mortgage-related securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	398,092
AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	1,266,510
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	543,620
AAA | FNMA/FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	78,125
AAA | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	245,675
AAA | GNMA commercial mortgage-related securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	398,092
AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	862,040
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	862,040
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 | GNMA commercial mortgage-related securities
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	20,883
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	20,883
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 | GNMA commercial mortgage-related securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities held to maturity, net	$ 0