Investment Securities - Credit Quality Indicators (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	$ 3,208.7	$ 3,453.7
AAA Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	3,041.5	3,260.7
AA Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	92.3	92.8
A Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	10.6	11.5
BBB Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	47.0	65.1
BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	5.0	10.0
Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	12.3	13.6
US Treasury Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	399.0	396.6
US Treasury Securities [Member] | AAA Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	399.0	396.6
US Treasury Securities [Member] | AA Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
US Treasury Securities [Member] | A Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
US Treasury Securities [Member] | BBB Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
US Treasury Securities [Member] | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
US Treasury Securities [Member] | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
State, county and municipal securities
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	179.2	181.2
State, county and municipal securities | AAA Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	69.0	68.3
State, county and municipal securities | AA Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	92.3	92.8
State, county and municipal securities | A Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	10.6	11.5
State, county and municipal securities | BBB Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
State, county and municipal securities | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
State, county and municipal securities | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	7.3	8.6
US Government Agencies Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	354.5	351.7
US Government Agencies Debt Securities [Member] | AAA Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	354.5	351.7
US Government Agencies Debt Securities [Member] | AA Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
US Government Agencies Debt Securities [Member] | A Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
US Government Agencies Debt Securities [Member] | BBB Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
US Government Agencies Debt Securities [Member] | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
US Government Agencies Debt Securities [Member] | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Corporate Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	57.0	80.1
Corporate Debt Securities [Member] | AAA Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Corporate Debt Securities [Member] | AA Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Corporate Debt Securities [Member] | A Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Corporate Debt Securities [Member] | BBB Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	47.0	65.1
Corporate Debt Securities [Member] | BB Rating
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	5.0	10.0
Corporate Debt Securities [Member] | Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	5.0	5.0
Commercial Mortgage Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	510.5	526.8
Commercial Mortgage Backed Securities | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	359.7	364.9
Commercial Mortgage Backed Securities | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	150.8	161.9
Commercial Mortgage Backed Securities | AAA Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	359.7	364.9
Commercial Mortgage Backed Securities | AAA Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	150.8	161.9
Commercial Mortgage Backed Securities | AA Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Commercial Mortgage Backed Securities | AA Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Commercial Mortgage Backed Securities | A Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Commercial Mortgage Backed Securities | A Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Commercial Mortgage Backed Securities | BBB Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Commercial Mortgage Backed Securities | BBB Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Commercial Mortgage Backed Securities | BB Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Commercial Mortgage Backed Securities | BB Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Commercial Mortgage Backed Securities | Not Rated | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Commercial Mortgage Backed Securities | Not Rated | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Residential Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,232.6	1,391.5
Residential Mortgage-Backed Securities | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,189.8	1,342.9
Residential Mortgage-Backed Securities | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	42.8	48.6
Residential Mortgage-Backed Securities | AAA Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	1,189.8	1,342.9
Residential Mortgage-Backed Securities | AAA Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	42.8	48.6
Residential Mortgage-Backed Securities | AA Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Residential Mortgage-Backed Securities | AA Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Residential Mortgage-Backed Securities | A Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Residential Mortgage-Backed Securities | A Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Residential Mortgage-Backed Securities | BBB Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Residential Mortgage-Backed Securities | BBB Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Residential Mortgage-Backed Securities | BB Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Residential Mortgage-Backed Securities | BB Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Residential Mortgage-Backed Securities | Not Rated | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Residential Mortgage-Backed Securities | Not Rated | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Collateralized Mortgage Obligations
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	475.9	525.8
Collateralized Mortgage Obligations | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	334.1	525.8
Collateralized Mortgage Obligations | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	141.8
Collateralized Mortgage Obligations | AAA Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	334.1	525.8
Collateralized Mortgage Obligations | AAA Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	141.8
Collateralized Mortgage Obligations | AA Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Collateralized Mortgage Obligations | AA Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0
Collateralized Mortgage Obligations | A Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Collateralized Mortgage Obligations | A Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0
Collateralized Mortgage Obligations | BBB Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Collateralized Mortgage Obligations | BBB Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0
Collateralized Mortgage Obligations | BB Rating | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	0.0
Collateralized Mortgage Obligations | BB Rating | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0
Collateralized Mortgage Obligations | Not Rated | FNMA and FHLMC
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	0.0	$ 0.0
Collateralized Mortgage Obligations | Not Rated | GNMA
Schedule of Held-to-maturity Securities [Line Items]
Debt Securities, Held-to-maturity	$ 0.0