Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Available-for-Sale:
Fair Value, Less than 12 Months	$ 5,830.5	$ 2,760.3
Gross Unrealized Losses, Less than 12 Months	(287.1)	(58.3)
Fair Value, 12 Months or Longer	1,331.9	68.7
Gross Unrealized Losses, 12 Months or Longer	(169.1)	(1.1)
Fair Value, Total	7,162.4	2,829.0
Gross Unrealized Losses, Total	(456.2)	(59.4)
US Treasury Notes Securities
Available-for-Sale:
Fair Value, Less than 12 Months	177.6
Gross Unrealized Losses, Less than 12 Months	(23.0)
Fair Value, 12 Months or Longer	458.1
Gross Unrealized Losses, 12 Months or Longer	(39.6)
Fair Value, Total	635.7
Gross Unrealized Losses, Total	(62.6)
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	136.7	278.7
Gross Unrealized Losses, Less than 12 Months	(15.8)	(9.1)
Fair Value, 12 Months or Longer	123.3	5.0
Gross Unrealized Losses, 12 Months or Longer	(27.1)	(0.2)
Fair Value, Total	260.0	283.7
Gross Unrealized Losses, Total	(42.9)	(9.3)
Obligations of U.S. government agencies
Available-for-Sale:
Fair Value, Less than 12 Months	191.7	297.0
Gross Unrealized Losses, Less than 12 Months	(6.5)	(8.9)
Fair Value, 12 Months or Longer	28.1	16.4
Gross Unrealized Losses, 12 Months or Longer	(2.9)	(0.3)
Fair Value, Total	219.8	313.4
Gross Unrealized Losses, Total	(9.4)	(9.2)
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Fair Value, Less than 12 Months	3,838.2	1,262.8
Gross Unrealized Losses, Less than 12 Months	(159.7)	(23.0)
Fair Value, 12 Months or Longer	621.8	26.4
Gross Unrealized Losses, 12 Months or Longer	(81.6)	(0.3)
Fair Value, Total	4,460.0	1,289.2
Gross Unrealized Losses, Total	(241.3)	(23.3)
Private mortgage-backed securities
Available-for-Sale:
Fair Value, Less than 12 Months	242.9	127.2
Gross Unrealized Losses, Less than 12 Months	(24.2)	(1.1)
Fair Value, 12 Months or Longer	10.1	0.0
Gross Unrealized Losses, 12 Months or Longer	(0.9)	0.0
Fair Value, Total	253.0	127.2
Gross Unrealized Losses, Total	(25.1)	(1.1)
Collateralized Loan Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	1,094.0
Gross Unrealized Losses, Less than 12 Months	(51.7)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	1,094.0
Gross Unrealized Losses, Total	(51.7)
Corporate Debt Securities
Available-for-Sale:
Fair Value, Less than 12 Months	149.4	109.9
Gross Unrealized Losses, Less than 12 Months	(6.2)	(3.3)
Fair Value, 12 Months or Longer	90.5	20.9
Gross Unrealized Losses, 12 Months or Longer	(17.0)	(0.3)
Fair Value, Total	239.9	130.8
Gross Unrealized Losses, Total	$ (23.2)	$ (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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