Summary of Long-Term Debt Outstanding (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Debt Disclosure [Line Items]
Federal Home Loan Bank Borrowings			$ 135.0
Capital Lease Obligations	31.8	[1]	36.9	[1]
Total Long-Term Debt	1,615.1		1,709.2
Long-Term Debt Qualifying as Risk-Based Capital	1,009.1		1,158.7
Notes 6.50 Percent Bank Subordinated Debt Due August 2018
Debt Disclosure [Line Items]
Subordinated Debt	335.0	[2],[3],[4],[5],[6]	342.4	[2],[3],[4],[5],[6]
Sterling Denominated Notes 5.375 Percent Bank Subordinated Debt Due March 2015
Debt Disclosure [Line Items]
Subordinated Debt	233.7	[2],[3],[7]	248.3	[2],[3],[7]
Notes 5.85 Percent Bank Subordinated Debt Due November 2017
Debt Disclosure [Line Items]
Subordinated Debt	221.6	[2],[3],[4],[5]	228.9	[2],[3],[4],[5]
Bank-Subordinated Debt
Debt Disclosure [Line Items]
Subordinated Debt	790.3	[2],[3]	819.6	[2],[3]
Corporation-Subordinated 3.95% Notes due Oct. 2025
Debt Disclosure [Line Items]
Subordinated Debt	793.0	[2],[3],[4],[8]	717.7	[2],[3],[4],[8]
Expiring One Year From Balance Sheet Date
Debt Disclosure [Line Items]
Federal Home Loan Bank Borrowings			$ 135.0

[1] Refer to Note 10 - Lease Commitments.

[2] Not redeemable prior to maturity.

[3] Debt issue costs are recorded as an asset and amortized on a straight-line basis over the life of the Note.

[4] Interest rate swap contracts were entered into to modify the interest expense on these subordinated notes from fixed rates to floating rates. The swaps are recorded as fair value hedges and at December 31, 2014, increases in the carrying values of subordinated notes outstanding of $100.6 million were recorded. As of December 31, 2013, net adjustments in the carrying values of subordinated notes outstanding of $40.1 million were recorded.

[5] Under the terms of its current Offering Circular dated November 6, 2013, the Bank has the ability to offer from time to time its senior bank notes in an aggregate principal amount of up to $4.5 billion at any one time outstanding and up to an additional $1.0 billion of subordinated notes. Each senior note will mature from 30 days to fifteen years, and each subordinated note will mature from five years to fifteen years, following its date of original issuance. Each note will mature on such date as selected by the initial purchaser and agreed to by the Bank.

[6] Notes issued at a discount of 0.02%

[7] Notes issued at a discount of 0.484%

[8] Notes issued at a discount of 0.114%