Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) $ in Millions	Dec. 31, 2023 USD ($) Investments	Dec. 31, 2022 USD ($) Investments
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	$ 3,208.7	$ 3,453.7
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities) | Investments	995	1,222
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 0.9	$ 5,071.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	5,505.5	1,545.6
Debt Securities, Available-for-sale, Unrealized Loss Position	5,506.4	6,617.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(469.5)	(629.7)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(346.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(469.5)	(283.1)
US Government Agencies Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	354.5	351.7
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.0	152.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	168.0	46.1
Debt Securities, Available-for-sale, Unrealized Loss Position	168.0	198.3
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(10.9)	(17.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(10.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(10.9)	(7.2)
Corporate Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	57.0	80.1
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.0	130.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	236.5	110.8
Debt Securities, Available-for-sale, Unrealized Loss Position	236.5	241.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(24.4)	(30.8)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(8.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(24.4)	(22.5)
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	133.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	210.5	94.9
Debt Securities, Available-for-sale, Unrealized Loss Position	210.5	228.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(30.9)	(36.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(19.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(30.9)	(17.8)
US Treasury notes
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	399.0	396.6
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.0	168.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	224.7	170.4
Debt Securities, Available-for-sale, Unrealized Loss Position	224.7	339.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(25.5)	(34.5)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(4.7)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(25.5)	(29.8)
State, county and municipal securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	179.2	181.2
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.0	104.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	217.1	146.1
Debt Securities, Available-for-sale, Unrealized Loss Position	217.1	251.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(36.9)	(50.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(9.0)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(36.9)	(41.6)
Collateralized Loan Obligations
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.0	1,082.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	935.7	28.9
Debt Securities, Available-for-sale, Unrealized Loss Position	935.7	1,111.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(2.3)	(33.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(33.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2.3)	(0.5)
Commercial Mortgage Backed Securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	510.5	526.8
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.2	1,069.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,083.1	270.8
Debt Securities, Available-for-sale, Unrealized Loss Position	1,083.3	1,340.6
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(87.7)	(116.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(59.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(87.7)	(57.0)
Residential Mortgage-Backed Securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	1,232.6	1,391.5
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.7	1,213.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,287.5	344.6
Debt Securities, Available-for-sale, Unrealized Loss Position	1,288.2	1,558.1
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(130.6)	(167.7)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(112.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(130.6)	(55.4)
Collateralized Mortgage Obligations
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	475.9	525.8
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.0	1,016.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,142.4	333.0
Debt Securities, Available-for-sale, Unrealized Loss Position	1,142.4	1,349.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(120.3)	(141.4)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	(90.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ (120.3)	$ (51.3)