Off-Balance-Sheet Financial Instruments - Commitments and Letters of Credit (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Commercial Letters of Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and letters of credit	$ 37.7	$ 24.0
Commitments to Extend Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and letters of credit	26,822.6	32,768.1
Commitments, participated to others	385.5	377.2
Standby Letters of Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and letters of credit	$ 2,970.0	$ 3,846.1
Weighted average maturity, letters of credit	22 months	24 months
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	$ 92.5	$ 134.2