Securities - Additional Information (Detail) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2012 Investment		Sep. 30, 2011		Sep. 30, 2012 Investment		Sep. 30, 2011		Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment security gains (losses), net	$ 217,500	[1]	$ (2,000,000)	[1]	$ (1,700,000)	[1]	$ (24,100,000)	[1]
Gross proceeds from sale of securities	218,200,000				2,700,000,000
Gross realized securities gains	356,000				23,500,000		1,600,000
Gross realized securities losses	1,000				21,900,000		877,600
Net Impairment Losses Recognized in Earnings	200,000		1,300,000		3,300,000		23,300,000
Realized securities gains (losses)			(706,800)
Number of securities in an unrealized loss position	215				215
Total Fair Value	2,222,400,000				2,222,400,000				10,223,500,000
Total Unrealized Losses	31,800,000				31,800,000				85,000,000
12 Months or Longer Unrealized Losses	21,700,000				21,700,000				51,600,000
Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	4,000,000				4,000,000
Others | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	8,600,000				8,600,000
Auction Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net Impairment Losses Recognized in Earnings					1,600,000
Total Unrealized Losses	4,300,000				4,300,000
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	400,000				400,000
Percent of corporate debt portfolio	40.00%				40.00%
Residential Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net Impairment Losses Recognized in Earnings	148,600		1,300,000		3,300,000		23,300,000
Total Unrealized Losses	13,300,000				13,300,000
Number of securities in an unrealized loss position for more than 12 months	15				15
12 Months or Longer Unrealized Losses	95,500,000				95,500,000
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	120,700,000				120,700,000
Residential mortgage-backed securities fair value	109,000,000				109,000,000
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	94.00%				94.00%
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	98.00%				98.00%
Other Asset-Backed | Floating Rate Securities | Upper Limit
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average life	5				5
Non-Agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net Impairment Losses Recognized in Earnings	$ 148,600		$ 1,300,000		$ 1,700,000		$ 23,300,000
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%

[1] Changes in Other-Than-Temporary-Impairment (OTTI) Losses $ 0.4 $ 0.5 $ (2.7 ) $ (1.1 ) Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI (0.6 ) (1.8 ) (0.6 ) (22.2 ) Other Security Gains (Losses), net 0.4 (0.7 ) 1.6 (0.8 ) Investment Security Gains (Losses), net $ 0.2 $ (2.0 ) $ (1.7 ) $ (24.1 )