Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) $ in Millions	Dec. 31, 2025 USD ($) Investments	Dec. 31, 2024 USD ($) Investments
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	$ 2,342.6	$ 2,688.4
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities) | Investments	633	670
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 258.1	$ 133.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3,616.2	4,047.2
Debt Securities, Available-for-sale, Unrealized Loss Position	3,874.3	4,180.6
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(238.0)	(420.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1.1)	(2.7)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(236.9)	(417.4)
US Government Agencies Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	462.3	470.1
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	2.4	3.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	195.8	206.2
Debt Securities, Available-for-sale, Unrealized Loss Position	198.2	209.8
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(4.2)	(10.8)
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	(4.2)	(10.8)
Corporate Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	29.9	57.0
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.0	27.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	150.0	205.5
Debt Securities, Available-for-sale, Unrealized Loss Position	150.0	233.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(8.7)	(16.7)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(0.5)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8.7)	(16.2)
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	174.3	190.4
Debt Securities, Available-for-sale, Unrealized Loss Position	174.3	190.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(19.5)	(27.6)
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	(19.5)	(27.6)
US Treasury notes
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity		99.8
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	237.7	226.8
Debt Securities, Available-for-sale, Unrealized Loss Position	237.7	226.8
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(8.8)	(18.1)
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	(8.8)	(18.1)
State, county and municipal securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	173.2	176.3
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	210.4	210.1
Debt Securities, Available-for-sale, Unrealized Loss Position	210.4	210.1
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(27.2)	(40.0)
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	(27.2)	(40.0)
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
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		0.0
Debt Securities, Available-for-sale, Unrealized Loss Position		10.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0.0
Commercial Mortgage Backed Securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	354.8	374.9
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1.3	38.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	821.6	934.9
Debt Securities, Available-for-sale, Unrealized Loss Position	822.9	973.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(36.1)	(74.5)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(0.4)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(36.1)	(74.1)
Residential Mortgage-Backed Securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	941.3	1,082.5
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	254.4	33.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	956.5	1,086.2
Debt Securities, Available-for-sale, Unrealized Loss Position	1,210.9	1,119.9
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(68.7)	(125.8)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1.1)	(1.5)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(67.6)	(124.3)
Collateralized Mortgage Obligations
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	381.1	427.8
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.0	20.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	869.9	987.1
Debt Securities, Available-for-sale, Unrealized Loss Position	869.9	1,007.6
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(64.8)	(106.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(0.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ (64.8)	$ (106.3)