Derivative Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Derivative Financial Instruments, exposure
Cash and securities posted under collateral arrangements associated with derivative credit support agreements	$ 435.2	$ 502.4
Aggregate fair value of all derivative instruments with credit-risk-related contingent features that were in a liability position	1,401.6	1,484.0
Collateral posted supporting derivatives with credit-risk-related contingent features that were in a liability position	435.2	502.4
Additional collateral required to be posted if derivative credit-risk-related contingent features were triggered	73.7
Cash collateral received associated with derivative credit support annex agreements	282.5	237.0
Notional amount	32,594.7	33,001.6
Gross credit exposure	1,190.2	1,245.1
Less: collateral received	287.4	237.0
Net credit exposure	902.8	1,008.1
Interest rate swaps
Derivative Financial Instruments, exposure
Notional amount	18,674.8	19,498.3
Gross credit exposure	780.3	752.2
Interest rate collars
Derivative Financial Instruments, exposure
Notional amount	500.0	500.0
Gross credit exposure	44.2	38.5
Interest rate futures
Derivative Financial Instruments, exposure
Notional amount	66.5	522.0
Swaptions
Derivative Financial Instruments, exposure
Notional amount	325.0	68.5
Gross credit exposure	1.3
Foreign currency swaps
Derivative Financial Instruments, exposure
Notional amount	3,624.3	3,919.8
Gross credit exposure	238.6	318.6
Currency forwards
Derivative Financial Instruments, exposure
Notional amount	193.6	147.3
Gross credit exposure	5.3	1.5
Equity options
Derivative Financial Instruments, exposure
Notional amount	1,726.0	1,608.4
Gross credit exposure	112.0	120.3
Equity futures
Derivative Financial Instruments, exposure
Notional amount	337.7	270.3
Credit default swaps
Derivative Financial Instruments, exposure
Notional amount	1,370.5	1,530.3
Gross credit exposure	8.5	14.0
Total return swaps
Derivative Financial Instruments, exposure
Notional amount	100.0	15.0
Embedded derivative financial instruments
Derivative Financial Instruments, exposure
Notional amount	$ 5,676.3	$ 4,921.7

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

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- Definition

The carrying value as of the balance sheet date of the cash received as collateral associated with derivative credit support annex agreements.

+ References

No definition available.

+ Details

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- Definition

The carrying value as of the balance sheet date of the cash and securities pledged as collateral associated with derivative credit support agreements.

+ References

No definition available.

+ Details

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- Definition

The carrying value as of the balance sheet date of the cash and securities received as collateral associated with derivative credit support agreements.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

The amount at risk if a counterparty to the derivative instruments does not perform.

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

The amount at risk if a counterparty to the derivative instrument does not perform, after considering collateral received.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44D

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44D

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959

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- Definition

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44D

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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