Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 Investment		Mar. 31, 2011		Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Security Gains (Losses), net	$ (2.4)	[1]	$ (5.5)	[1]
Net Impairment Losses Recognized in Earnings	3.1		5.1
Realized securities gains (losses)	0.7		(0.4)
Number of securities in an unrealized loss position	351
Total Fair Value	10,545.8				10,223.5
Total Unrealized Losses	66.8				85.0
12 Months or Longer Unrealized Losses	47.6				51.6
Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	12.9
Others | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	9.3
Auction Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net Impairment Losses Recognized in Earnings	1.6
Total Unrealized Losses	12.3
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	1.3
Percent of corporate debt portfolio	60.00%
Residential Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net Impairment Losses Recognized in Earnings	(3.1)		(5.1)
Total Unrealized Losses	23.9
Number of securities in an unrealized loss position for more than 12 months	15
12 Months or Longer Unrealized Losses	23.0
Residential Mortgage-Backed | Sub-prime and Alt-A Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Credit Rating	Below double-A
Residential mortgage-backed securities total amortized cost	140.5
Residential mortgage-backed securities fair value	117.2
Residential Mortgage-Backed | AA Credit Rating | Sub-prime and Alt-A Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percentage of residential mortgage backed securities rated below double-A	85.00%
Other Asset-Backed | Floating Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Credit Rating	AAA
Percentage of "other asset-backed" securities rated triple-A	100.00%
Other Asset-Backed | Floating Rate Securities | Upper Limit
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average life	5
Non-Agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net Impairment Losses Recognized in Earnings	$ 1.5		$ 5.1
Non-Agency RMBS | Financing Receivable, Current | Upper Limit
Schedule of Trading Securities and Other Trading Assets [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	30.00%
Non-Agency RMBS | Financing Receivable, Current | Lower Limit
Schedule of Trading Securities and Other Trading Assets [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	5.00%
Non-Agency RMBS | Financing Receivable, 30 to 59 Days Past Due
Schedule of Trading Securities and Other Trading Assets [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	30.00%
Non-Agency RMBS | Financing Receivable, 60 to 89 Days Past Due
Schedule of Trading Securities and Other Trading Assets [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	80.00%
Non-Agency RMBS | Financing Receivable, 90 Days and Greater Past Due
Schedule of Trading Securities and Other Trading Assets [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	90.00%
Non-Agency RMBS | Other Real Estate Owned and Loans in Foreclosure
Schedule of Trading Securities and Other Trading Assets [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	100.00%

[1] Changes in Other-Than-Temporary-Impairment (OTTI) Losses $ (3.1 ) $ 0.1 Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI - (5.2 ) Other Security Gains (Losses), net 0.7 (0.4 ) Investment Security Gains (Losses), net $ (2.4 ) $ (5.5 )