Investment Securities - Credit Quality Indicators (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	$ 3,260.3	$ 3,453.7
AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	3,092.4	3,260.7
AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	92.3	92.8
A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	10.6	11.5
BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	47.0	65.1
Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	13.0	13.6
BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	5.0	10.0
U.S. Treasury notes
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	398.4	396.6
U.S. Treasury notes | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	398.4	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	179.0	181.2
State, county, and municipal securities | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	68.1	68.3
State, county, and municipal securities | AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	92.3	92.8
State, county, and municipal securities | A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	10.6	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	8.0	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	353.8	351.7
Obligations of U.S. government agencies | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	353.8	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	57.0	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	47.0	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	5.0	10.0
Commercial Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	515.2	526.8
Commercial Mortgage-Backed Securities | FNMA/FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	360.9	364.9
Commercial Mortgage-Backed Securities | FNMA/FHLMC | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	360.9	364.9
Commercial Mortgage-Backed Securities | FNMA/FHLMC | AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Commercial Mortgage-Backed Securities | FNMA/FHLMC | A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Commercial Mortgage-Backed Securities | FNMA/FHLMC | BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Commercial Mortgage-Backed Securities | FNMA/FHLMC | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Commercial Mortgage-Backed Securities | FNMA/FHLMC | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Commercial Mortgage-Backed Securities | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	154.3	161.9
Commercial Mortgage-Backed Securities | GNMA | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	154.3	161.9
Commercial Mortgage-Backed Securities | GNMA | AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Commercial Mortgage-Backed Securities | GNMA | A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Commercial Mortgage-Backed Securities | GNMA | BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Commercial Mortgage-Backed Securities | GNMA | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Commercial Mortgage-Backed Securities | GNMA | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Residential Mortgage Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	1,269.8	1,391.5
Residential Mortgage Backed Securities | FNMA/FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	1,225.6	1,342.9
Residential Mortgage Backed Securities | FNMA/FHLMC | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	1,225.6	1,342.9
Residential Mortgage Backed Securities | FNMA/FHLMC | AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Residential Mortgage Backed Securities | FNMA/FHLMC | A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Residential Mortgage Backed Securities | FNMA/FHLMC | BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Residential Mortgage Backed Securities | FNMA/FHLMC | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Residential Mortgage Backed Securities | FNMA/FHLMC | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Residential Mortgage Backed Securities | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	44.2	48.6
Residential Mortgage Backed Securities | GNMA | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	44.2	48.6
Residential Mortgage Backed Securities | GNMA | AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Residential Mortgage Backed Securities | GNMA | A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Residential Mortgage Backed Securities | GNMA | BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Residential Mortgage Backed Securities | GNMA | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Residential Mortgage Backed Securities | GNMA | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0	0.0
Collateralized Mortgage Obligations
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	487.1	525.8
Collateralized Mortgage Obligations | FNMA/FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	342.0	525.8
Collateralized Mortgage Obligations | FNMA/FHLMC | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	342.0	525.8
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
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
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
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
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
Collateralized Mortgage Obligations | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	145.1
Collateralized Mortgage Obligations | GNMA | AAA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	145.1
Collateralized Mortgage Obligations | GNMA | AA
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0
Collateralized Mortgage Obligations | GNMA | A
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0
Collateralized Mortgage Obligations | GNMA | BBB
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0
Collateralized Mortgage Obligations | GNMA | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	0.0
Collateralized Mortgage Obligations | GNMA | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	$ 0.0