Securities - Additional Information (Detail) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2014 Investment		Sep. 30, 2013		Sep. 30, 2014 Investment		Sep. 30, 2013		Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Security Gains (Losses), net	$ 300,000	[1]	$ (2,200,000)	[1]	$ (3,300,000)	[1]	$ (1,900,000)	[1]
Gross proceeds from sale of securities	337,300,000		316,400,000		801,200,000		398,100,000
Gross realized securities gains	1,000,000		100,000		1,700,000		400,000
Gross realized securities losses	700,000		2,300,000		1,100,000		2,300,000
Number of securities in an unrealized loss position	471				471
Total Fair Value	9,852,500,000				9,852,500,000				10,250,000,000
Total Unrealized Losses	82,500,000				82,500,000				180,400,000
Net impairment losses recognized in earnings	0		0		3,900,000		0
U.S. Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	25,200,000				25,200,000
Others | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	16,300,000				16,300,000
Auction Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	500,000				500,000
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	33,400,000				33,400,000
Percent of corporate debt portfolio	40.00%				40.00%
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, 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, 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	$ 1,300,000				$ 1,300,000
Number of securities in an unrealized loss position for more than 12 months	2				2
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	100.00%				100.00%
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 0.3 (2.2 ) 0.6 (1.9 ) Investment Security Gains (Losses), net $ 0.3 $ (2.2 ) $ (3.3 ) $ (1.9 )