Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Investment		Mar. 31, 2012		Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment security gains (losses), net	$ 0.2	[1]	$ (2.4)	[1]
Net Impairment Losses Recognized in Earnings			3.1
Gross proceeds from sale of securities	55.7		1,300.0
Gross realized securities gains	0.2		1.6
Gross realized securities losses			0.9
Realized securities gains (losses)	0.2		0.7
Number of securities in an unrealized loss position	245
Total Fair Value	3,910.5				3,825.8
Total Unrealized Losses	25.5				30.2
12 Months or Longer Unrealized Losses	16.3				24.0
Net Impairment Losses Recognized in Earnings	0		3.1
U.S. Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	4.6
Others | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	7.0
Auction Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	1.4
Net Impairment Losses Recognized in Earnings			1.6
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	3.3
Percent of corporate debt portfolio	47.00%
Non-Agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net Impairment Losses Recognized in Earnings			1.5
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, 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, 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%
Residential Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	7.9
Number of securities in an unrealized loss position for more than 12 months	9
12 Months or Longer Unrealized Losses	68.1
Residential Mortgage-Backed | Sub-prime and Alt-A Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Residential mortgage-backed securities total amortized cost	78.7
Residential mortgage-backed securities fair value	$ 70.9
Other Asset-Backed | Floating Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percentage of "other asset-backed" securities rated triple-A	99.00%
Credit Rating	AAA
Other Asset-Backed | Floating Rate Securities | Upper Limit
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average life	5

[1] Changes in Other-Than-Temporary-Impairment (OTTI) Losses $ - $ (3.1 ) Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI - - Other Security Gains (Losses), net 0.2 0.7 Investment Security Gains (Losses), net $ 0.2 $ (2.4 )