Commitments and Letters of Credit (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Commitments to Extend Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Legally Binding Commitments	$ 481.4		$ 418.5
Commitments and Letters of Credit	35,127.6	[1]	32,174.8	[1]
Standby Letters of Credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commitments and Letters of Credit	4,468.1	[2]	4,451.1	[2]
Weighted average maturity of standby letters of credit	27 months		25 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	$ 221.4		$ 208.9

[1] These amounts exclude $481.4 million and $418.5 million of commitments participated to others at December 31, 2014 and 2013, respectively.

[2] These amounts include $221.4 million and $208.9 million of standby letters of credit secured by cash deposits or participated to others as of December 31, 2014 and 2013, respectively. The weighted average maturity of standby letters of credit was 27 months at December 31, 2014 and 25 months at December 31, 2013.