Investment Securities - Investment Securities by Credit Rating Type (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Investment securities - HTM, at amortized cost; fair value of $516,261 at December 31, 2019 and $298,648 at December 31, 2018	$ 1,107.1	$ 568.8
Investment securities - equity	158.5	167.3
Debt Securities, Available-for-sale	6,188.8	4,708.5
Residential mortgage-backed securities issued by GSEs
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	1,993.4	1,486.6
Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	68.5	84.6
Private label residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - HTM, at amortized cost; fair value of $516,261 at December 31, 2019 and $298,648 at December 31, 2018	216.9
Debt Securities, Available-for-sale	1,508.0	1,476.9
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	13.0
Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	113.9	113.9
Corporate debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	382.9	270.2
CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	44.6	53.4
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	926.2	146.9
Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - HTM, at amortized cost; fair value of $516,261 at December 31, 2019 and $298,648 at December 31, 2018	890.2	568.8
Debt Securities, Available-for-sale	1,215.1	1,187.4
Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	$ 81.7	$ 55.9

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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