Off-Balance-Sheet Financial Instruments - Commitments and Letters of Credit (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Legally Binding Commitments to Extend Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and letters of credit	$ 25,023.0	$ 26,822.6
Commitments, participated to others	242.3	385.5
Standby Letters of Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and letters of credit	$ 2,486.2	$ 2,970.0
Weighted average maturity, letters of credit	23 months	22 months
Commercial Letters of Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and letters of credit	$ 32.3	$ 37.7
Secured by Cash Deposits or Participated to Others | Standby Letters of Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and letters of credit	$ 72.3	$ 92.5