Off-Balance-Sheet Financial Instruments, Guarantees and Other Commitments - Commitments and Letters of Credit (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Legally Binding Commitments to Extend Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and letters of credit	$ 24,406.2	$ 25,023.0
Standby Letters of Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and letters of credit	2,416.7	2,486.2
Standby letters of credit, expiring next twelve months	1,400.0
Standby letters of credit expiring within 1 to 5 years	845.9
Commercial Letters of Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and letters of credit	32.3	32.3
Custody Securities Lent with Indemnification
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and letters of credit	138,085.9	128,904.8
Standby Letters of Credit | Secured by Cash Deposits or Participated to Others
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and letters of credit	44.5	72.3
Legally Binding Commitments to Extend Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments, participated to others	$ 243.6	$ 242.3