Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) $ in Millions	Dec. 31, 2022 USD ($) Investments	Dec. 31, 2021 USD ($) Investments
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	$ 3,453.7	$ 1,687.6
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities) | Investments	1,222	285
Less than 12 Months	$ 1,923.9	$ 1,067.7
Less than 12 Months	(187.2)	(35.7)
12 Months or More	1,075.2	0.0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(212.8)	0.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	2,999.1	1,067.7
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	(400.0)	(35.7)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	5,071.9	2,760.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	1,545.6	68.7
Debt Securities, Available-for-sale, Unrealized Loss Position	6,617.5	2,829.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(629.7)	(59.4)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(346.6)	(58.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(283.1)	(1.1)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	2,444.1	1,609.0
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	1,349.4	1,038.7
Less than 12 Months	(156.1)	(35.3)
12 Months or More	762.5	0.0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(145.5)	0.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	2,111.9	1,038.7
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	(301.6)	(35.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	3,299.7	1,262.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	948.4	26.4
Debt Securities, Available-for-sale, Unrealized Loss Position	4,248.1	1,289.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(426.0)	(23.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(262.3)	(23.0)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(163.7)	(0.3)
US Government Agencies Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	351.7
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	86.1
Less than 12 Months	(9.9)
12 Months or More	216.0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(39.7)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	302.1
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	(49.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	152.2	297.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	46.1	16.4
Debt Securities, Available-for-sale, Unrealized Loss Position	198.3	313.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(17.3)	(9.2)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(10.1)	(8.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(7.2)	(0.3)
Corporate Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	80.1	11.0
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	68.2
Less than 12 Months	(7.0)
12 Months or More	0.0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	68.2
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	(7.0)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	130.6	109.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	110.8	20.9
Debt Securities, Available-for-sale, Unrealized Loss Position	241.4	130.8
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(30.8)	(3.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(8.3)	(3.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(22.5)	(0.3)
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	133.1	127.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	94.9	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position	228.0	127.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(36.9)	(1.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(19.1)	(1.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(17.8)	0.0
US Treasury notes
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	396.6
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	168.8	684.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	170.4	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position	339.2	684.7
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(34.5)	(12.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(4.7)	(12.9)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(29.8)	0.0
State, county and municipal securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	181.2	67.6
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	33.8	29.0
Less than 12 Months	(4.0)	(0.4)
12 Months or More	96.7	0.0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(27.6)	0.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	130.5	29.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	(31.6)	(0.4)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	104.9	278.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	146.1	5.0
Debt Securities, Available-for-sale, Unrealized Loss Position	251.0	283.7
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(50.6)	(9.3)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(9.0)	(9.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(41.6)	$ (0.2)
Collateralized Loan Obligations
Available-for-Sale and Held-to-Maturity:
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,082.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	28.9
Debt Securities, Available-for-sale, Unrealized Loss Position	1,111.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(33.6)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(33.1)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(0.5)
US Treasury Notes Securities
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	386.4
Less than 12 Months	(10.2)
12 Months or More	0.0
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value	386.4
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	$ (10.2)