Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Available-for-Sale:
Fair Value, Less than 12 Months	$ 5,554.5	$ 2,760.3
Gross Unrealized Losses, Less than 12 Months	(421.7)	(58.3)
Fair Value, 12 Months or Longer	1,201.7	68.7
Gross Unrealized Losses, 12 Months or Longer	(243.3)	(1.1)
Fair Value, Total	6,756.2	2,829.0
Gross Unrealized Losses, Total	(665.0)	(59.4)
US Treasury Notes Securities
Available-for-Sale:
Fair Value, Less than 12 Months	168.2
Gross Unrealized Losses, Less than 12 Months	(5.1)
Fair Value, 12 Months or Longer	168.6
Gross Unrealized Losses, 12 Months or Longer	(31.6)
Fair Value, Total	336.8
Gross Unrealized Losses, Total	(36.7)
U.S. Treasury notes
Available-for-Sale:
Fair Value, Less than 12 Months		684.7
Gross Unrealized Losses, Less than 12 Months		(12.9)
Fair Value, 12 Months or Longer		0.0
Gross Unrealized Losses, 12 Months or Longer		0.0
Fair Value, Total		684.7
Gross Unrealized Losses, Total		(12.9)
State, county, and municipal securities
Available-for-Sale:
Fair Value, Less than 12 Months	110.6	278.7
Gross Unrealized Losses, Less than 12 Months	(14.4)	(9.1)
Fair Value, 12 Months or Longer	136.4	5.0
Gross Unrealized Losses, 12 Months or Longer	(42.9)	(0.2)
Fair Value, Total	247.0	283.7
Gross Unrealized Losses, Total	(57.3)	(9.3)
Obligations of U.S. government agencies
Available-for-Sale:
Fair Value, Less than 12 Months	152.4	297.0
Gross Unrealized Losses, Less than 12 Months	(10.9)	(8.9)
Fair Value, 12 Months or Longer	45.9	16.4
Gross Unrealized Losses, 12 Months or Longer	(6.4)	(0.3)
Fair Value, Total	198.3	313.4
Gross Unrealized Losses, Total	(17.3)	(9.2)
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Fair Value, Less than 12 Months	3,697.1	1,262.8
Gross Unrealized Losses, Less than 12 Months	(308.8)	(23.0)
Fair Value, 12 Months or Longer	699.8	26.4
Gross Unrealized Losses, 12 Months or Longer	(133.9)	(0.3)
Fair Value, Total	4,396.9	1,289.2
Gross Unrealized Losses, Total	(442.7)	(23.3)
Private mortgage-backed securities
Available-for-Sale:
Fair Value, Less than 12 Months	206.1	127.2
Gross Unrealized Losses, Less than 12 Months	(32.3)	(1.1)
Fair Value, 12 Months or Longer	27.0	0.0
Gross Unrealized Losses, 12 Months or Longer	(4.8)	0.0
Fair Value, Total	233.1	127.2
Gross Unrealized Losses, Total	(37.1)	(1.1)
Collateralized Loan Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	1,076.1
Gross Unrealized Losses, Less than 12 Months	(39.8)
Fair Value, 12 Months or Longer	29.0
Gross Unrealized Losses, 12 Months or Longer	(0.6)
Fair Value, Total	1,105.1
Gross Unrealized Losses, Total	(40.4)
Corporate Debt Securities
Available-for-Sale:
Fair Value, Less than 12 Months	144.0	109.9
Gross Unrealized Losses, Less than 12 Months	(10.4)	(3.3)
Fair Value, 12 Months or Longer	95.0	20.9
Gross Unrealized Losses, 12 Months or Longer	(23.1)	(0.3)
Fair Value, Total	239.0	130.8
Gross Unrealized Losses, Total	$ (33.5)	$ (3.6)

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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