Summary of Long-Term Debt Outstanding (Parenthetical) (Detail) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 06, 2013
Debt Disclosure [Line Items]
Increases in the carrying values of notes outstanding covered by interest-rate swap contracts	$ 0	$ (900,000)	$ 400,000
Notes 6.50 Percent Bank Subordinated Debt Due August 2018
Debt Disclosure [Line Items]
Interest Rate	6.50%	6.50%
Notes issued at a discount	0.02%	0.02%
Sterling Denominated Notes 5.375 Percent Bank Subordinated Debt Due March 2015
Debt Disclosure [Line Items]
Interest Rate	5.375%	5.375%
Notes issued at a discount	0.484%	0.484%
Notes 5.85 Percent Bank Subordinated Debt Due November 2017
Debt Disclosure [Line Items]
Interest Rate	5.85%	5.85%
Corporation-Subordinated 3.95% Notes due Oct. 2025
Debt Disclosure [Line Items]
Interest Rate	3.95%	3.95%
Notes issued at a discount	0.114%	0.114%
Expiring One Year From Balance Sheet Date
Debt Disclosure [Line Items]
Federal Home Loan Bank Borrowings, Average Rate at Year End		4.40%
Interest Rate Swaps | Subordinated Debt
Debt Disclosure [Line Items]
Increases in the carrying values of notes outstanding covered by interest-rate swap contracts	100,600,000	40,100,000
Upper Limit | Notes 6.50 Percent Bank Subordinated Debt Due August 2018
Debt Disclosure [Line Items]
Debt maturity				15 years
Upper Limit | Notes 5.85 Percent Bank Subordinated Debt Due November 2017
Debt Disclosure [Line Items]
Debt maturity				15 years
Upper Limit | Senior Notes
Debt Disclosure [Line Items]
Offering Circular, ability to offer notes in aggregate principal amount				4,500,000,000
Debt maturity				15 years
Upper Limit | Subordinated Debt
Debt Disclosure [Line Items]
Offering Circular, ability to offer notes in aggregate principal amount				1,000,000,000
Lower Limit | Notes 6.50 Percent Bank Subordinated Debt Due August 2018
Debt Disclosure [Line Items]
Debt maturity				5 years
Lower Limit | Notes 5.85 Percent Bank Subordinated Debt Due November 2017
Debt Disclosure [Line Items]
Debt maturity				5 years
Lower Limit | Senior Notes
Debt Disclosure [Line Items]
Debt maturity				30 days