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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Name Statement of Financial Accounting Standard (FAS)

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Name Regulation S-X (SX)

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-Section 04

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2011 Contract Investment	Sep. 30, 2010	Sep. 30, 2011 Contract Investment	Sep. 30, 2010	Dec. 31, 2010
Gain (Loss) on Investments [Line Items]
Realized securities losses	$ 2.0	$ 13.5	$ 24.1	$ 13.3
Realized securities (gains) losses	0.7	0.5	0.8	0.8
Net Impairment Losses Recognized in Earnings	1.3	14.0	23.3	14.1
Number of securties in an unrealized loss position	341		341
Total Fair Value	9,676.4		9,676.4		4,600.6
Total Unrealized Losses	74.0		74.0		99.5
12 Months or Longer Unrealized Losses	51.3		51.3		65.3
Residential Mortgage-Backed
Gain (Loss) on Investments [Line Items]
Net Impairment Losses Recognized in Earnings	1.3	14.0	23.3	14.1
Number of securties in an unrealized loss position for more than 12 months	25		25
Total Unrealized Losses	32.4		32.4
12 Months or Longer Unrealized Losses	32.2		32.2
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	81.00%		81.00%
Residential Mortgage-Backed \| Sub-prime and Alt-A Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Net Impairment Losses Recognized in Earnings	1.3	14.0	23.3	14.1
Credit Rating			Below double-A
Residential mortgage-backed securities total amortized cost	179.7		179.7
Residential mortgage-backed securities fair value	149.2		149.2
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%
Government Sponsored Agency
Gain (Loss) on Investments [Line Items]
Total Unrealized Losses	13.4		13.4
Other \| Community Reinvestment Act CRA
Gain (Loss) on Investments [Line Items]
Total Unrealized Losses	12.7		12.7
Auction Rate
Gain (Loss) on Investments [Line Items]
Total Unrealized Losses	9.8		9.8
Corporate Debt
Gain (Loss) on Investments [Line Items]
Total Unrealized Losses	$ 2.4		$ 2.4
Percent of corporate debt unrealized losses	74.00%		74.00%