Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) $ in Millions	Dec. 31, 2024 USD ($) Investments	Dec. 31, 2023 USD ($) Investments
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	$ 2,688.4	$ 3,208.7
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities) | Investments	670	704
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 133.4	$ 0.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	4,047.2	5,505.5
Debt Securities, Available-for-sale, Unrealized Loss Position	4,180.6	5,506.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(420.1)	(469.5)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2.7)	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(417.4)	(469.5)
US Government Agencies Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	470.1	354.5
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	3.6	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	206.2	168.0
Debt Securities, Available-for-sale, Unrealized Loss Position	209.8	168.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(10.8)	(10.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(10.8)	(10.9)
Corporate Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	57.0	57.0
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	27.5	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	205.5	236.5
Debt Securities, Available-for-sale, Unrealized Loss Position	233.0	236.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(16.7)	(24.4)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(0.5)	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(16.2)	(24.4)
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	190.4	210.5
Debt Securities, Available-for-sale, Unrealized Loss Position	190.4	210.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(27.6)	(30.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(27.6)	(30.9)
US Treasury notes
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	99.8	399.0
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	226.8	224.7
Debt Securities, Available-for-sale, Unrealized Loss Position	226.8	224.7
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(18.1)	(25.5)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(18.1)	(25.5)
State, county and municipal securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	176.3	179.2
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	210.1	217.1
Debt Securities, Available-for-sale, Unrealized Loss Position	210.1	217.1
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(40.0)	(36.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(40.0)	(36.9)
Collateralized Loan Obligations
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	10.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0.0	935.7
Debt Securities, Available-for-sale, Unrealized Loss Position	10.0	935.7
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	0.0	(2.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	(2.3)
Commercial Mortgage Backed Securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	374.9	510.5
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	38.1	0.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	934.9	1,083.1
Debt Securities, Available-for-sale, Unrealized Loss Position	973.0	1,083.3
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(74.5)	(87.7)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(0.4)	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(74.1)	(87.7)
Residential Mortgage-Backed Securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	1,082.5	1,232.6
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	33.7	0.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,086.2	1,287.5
Debt Securities, Available-for-sale, Unrealized Loss Position	1,119.9	1,288.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(125.8)	(130.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1.5)	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(124.3)	(130.6)
Collateralized Mortgage Obligations
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	427.8	475.9
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	20.5	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	987.1	1,142.4
Debt Securities, Available-for-sale, Unrealized Loss Position	1,007.6	1,142.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(106.6)	(120.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(0.3)	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ (106.3)	$ (120.3)