Investment Securities - Credit Quality Indicators (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	$ 3,376.9	$ 3,453.7
AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	3,207.1	3,260.7
AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	92.7	92.8
A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	11.5	11.5
BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	52.1	65.1
Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	13.5	13.6
BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	10.0
U.S. Treasury notes
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	397.2	396.6
U.S. Treasury notes | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	397.2	396.6
U.S. Treasury notes | AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. Treasury notes | A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. Treasury notes | BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. Treasury notes | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. Treasury notes | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
State, county, and municipal securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	180.9	181.2
State, county, and municipal securities | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	68.2	68.3
State, county, and municipal securities | AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	92.7	92.8
State, county, and municipal securities | A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	11.5	11.5
State, county, and municipal securities | BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
State, county, and municipal securities | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	8.5	8.6
State, county, and municipal securities | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Obligations of U.S. government agencies
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	352.3	351.7
Obligations of U.S. government agencies | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	352.3	351.7
Obligations of U.S. government agencies | AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Obligations of U.S. government agencies | A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Obligations of U.S. government agencies | BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Obligations of U.S. government agencies | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Obligations of U.S. government agencies | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Corporate securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	57.1	80.1
Corporate securities | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Corporate securities | AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Corporate securities | A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Corporate securities | BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	52.1	65.1
Corporate securities | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	5.0	5.0
Corporate securities | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	10.0
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	2,389.4	2,444.1
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | FNMA/FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	2,183.2	2,233.6
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | FNMA/FHLMC | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	2,183.2	2,233.6
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | FNMA/FHLMC | AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | FNMA/FHLMC | A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | FNMA/FHLMC | BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | FNMA/FHLMC | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | FNMA/FHLMC | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	206.2	210.5
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | GNMA | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	206.2	210.5
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | GNMA | AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | GNMA | A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | GNMA | BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | GNMA | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations | GNMA | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	$ 0.0	$ 0.0