Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014 Investment		Jun. 30, 2013		Jun. 30, 2014 Investment		Jun. 30, 2013		Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Security Gains (Losses), net	$ 0.4	[1]	$ 0.1	[1]	$ (3.6)	[1]	$ 0.3	[1]
Gross proceeds from sale of securities	264.2		26.0		463.9		81.7
Gross realized securities gains	0.4		0.1		0.7		300.0
Gross realized securities losses					0.4
Number of securities in an unrealized loss position	382				382
Total Fair Value	8,002.8				8,002.8				10,250.0
Total Unrealized Losses	81.4				81.4				180.4
12 Months or Longer Unrealized Losses	55.3				55.3				34.6
Net impairment losses recognized in earnings	0		0		3.9		0
U.S. Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	23.6				23.6
Others | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	13.6				13.6
Auction Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	0.7				0.7
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	31.2				31.2
Percent of corporate debt portfolio	44.00%				44.00%
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%				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%				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%				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%				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%				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%				100.00%
Residential Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	2.0				2.0
Number of securities in an unrealized loss position for more than 12 months	4				4
12 Months or Longer Unrealized Losses	23.1				23.1
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	44.5				44.5
Residential mortgage-backed securities fair value	$ 42.9				$ 42.9
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%				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 years

[1] Changes in Other-Than-Temporary-Impairment (OTTI) Losses $ - $ - $ (4.6 ) $ - Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI - - 0.7 - Other Security Gains (Losses), net 0.4 0.1 0.3 0.3 Investment Security Gains (Losses), net $ 0.4 $ 0.1 $ (3.6 ) $ 0.3