Investment Securities - Unrealized Losses and Fair Value of Investment Securities in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 67.9	$ 4.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	3,046.3	398.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	13.4	11.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	169.5	120.8
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	81.3	15.5
Debt Securities, Available-for-sale, Unrealized Loss Position	3,215.8	519.7
Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	141.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	1.2
Debt Securities, Available-for-sale, Unrealized Loss Position	141.4
Corporate debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9.0	0.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	107.0	17.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	5.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0.0	94.3
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	9.0	5.7
Debt Securities, Available-for-sale, Unrealized Loss Position	107.0	111.6
Residential mortgage-backed securities issued by GSEs
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	32.6	3.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,355.7	231.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	8.8	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	142.1	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	41.4	3.8
Debt Securities, Available-for-sale, Unrealized Loss Position	1,497.8	231.9
Private label residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	24.2	0.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,250.0	149.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0.0	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	24.2	0.5
Debt Securities, Available-for-sale, Unrealized Loss Position	1,250.0	149.7
Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	19.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	0.6
Debt Securities, Available-for-sale, Unrealized Loss Position	19.5
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	170.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	0.3
Debt Securities, Available-for-sale, Unrealized Loss Position	170.7
Other Security Investments
Debt Securities, Available-for-sale [Line Items]
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, Less than 12 Months	2.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4.6	5.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	27.4	26.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	4.6	5.5
Debt Securities, Available-for-sale, Unrealized Loss Position	$ 29.4	$ 26.5

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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-Name Accounting Standards Codification
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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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