Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011 Investment	Jun. 30, 2010	Jun. 30, 2011 Investment	Jun. 30, 2010	Dec. 31, 2010
Gain (Loss) on Investments [Line Items]
Realized securities losses	$ 16.6	$ 0.1	$ 22.1	$ 0.1
Realized securities (gains) losses	0.3	0	0.1	0.3
Net Impairment Losses Recognized in Earnings	16.9	0.1	22.0	0.1
Number of securties in an unrealized loss position	286		286
Total Fair Value	3,486.9		3,486.9		4,600.6
Total Unrealized Losses	63.2		63.2		99.5
12 Months or Longer Unrealized Losses	46.8		46.8		65.3
Government Sponsored Agency
Gain (Loss) on Investments [Line Items]
Total Unrealized Losses	8.6		8.6
Other | Community Reinvestment Act CRA
Gain (Loss) on Investments [Line Items]
Total Unrealized Losses	14.9		14.9
Auction Rate
Gain (Loss) on Investments [Line Items]
Total Unrealized Losses	5.4		5.4
Corporate Debt
Gain (Loss) on Investments [Line Items]
Total Unrealized Losses	0.3		0.3
Percent of corporate debt unrealized losses	83.00%		83.00%
Residential Mortgage-Backed
Gain (Loss) on Investments [Line Items]
Net Impairment Losses Recognized in Earnings	16.9		22.0
Number of securties in an unrealized loss position for more than 12 months	24		24
12 Months or Longer Unrealized Losses	31.4		31.4
Residential Mortgage-Backed | AA Credit Rating | Sub-prime and Alt-A Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Percentage of residential mortgage backed securities rated below double-A	80.00%		80.00%
Residential Mortgage-Backed | Sub-prime and Alt-A Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Net Impairment Losses Recognized in Earnings	16.9	0.1	22.0	0.1
Credit Rating			Below double-A
Residential mortgage-backed securities total amortized cost	196.1		196.1
Residential mortgage-backed securities fair value	$ 166.7		$ 166.7
Other Asset-Backed | Floating Rate Securities
Gain (Loss) on Investments [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
Gain (Loss) on Investments [Line Items]
Average life	5		5
Non-Agency RMBS | Financing Receivable, Current | Upper Limit
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	100.00%		100.00%