Derivative Financial Instruments - Notional Amounts and Credit Exposure (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Derivative Financial Instruments, exposure
Cash and securities posted under collateral arrangements associated with derivative credit support agreements and Futures Commission Merchant agreements	$ 342.7	$ 271.6
Aggregate fair value of all derivative instruments with credit-risk-related contingent features that were in a liability position	606.5	656.2
Collateral and initial margin posted supporting derivatives with credit-risk-related contingent features that were in a liability position	342.7	271.6
Additional collateral required to be posted if derivative credit-risk-related contingent features were triggered	75.8
Cash collateral received associated with derivative credit support annex agreements and Futures Commission Merchant agreements	217.5	148.3
Notional amount	45,028.4	41,098.5
Gross credit exposure	703.4	693.8
Less: collateral received	234.2	183.5
Net credit exposure	469.2	510.3
Interest rate swaps
Derivative Financial Instruments, exposure
Cash exchanged under contract	0.0
Principal payments made under contract	0.0
Notional amount	21,704.2	19,182.6
Gross credit exposure	505.5	510.8
Interest rate options
Derivative Financial Instruments, exposure
Notional amount	4,900.0	4,900.0
Gross credit exposure	34.1	41.0
Swaptions
Derivative Financial Instruments, exposure
Notional amount	259.0	260.0
Interest rate futures
Derivative Financial Instruments, exposure
Notional amount	162.0	147.5
Currency swaps
Derivative Financial Instruments, exposure
Notional amount	1,751.0	1,975.5
Gross credit exposure	105.6	97.1
Currency forwards
Derivative Financial Instruments, exposure
Notional amount	1,040.6	270.7
Gross credit exposure	4.4	1.4
Equity options
Derivative Financial Instruments, exposure
Notional amount	3,604.8	3,293.4
Gross credit exposure	39.9	30.2
Equity futures
Derivative Financial Instruments, exposure
Notional amount	514.2	498.1
Credit default swaps
Derivative Financial Instruments, exposure
Notional amount	1,084.5	1,234.5
Gross credit exposure	13.4	13.3
Total return swaps
Derivative Financial Instruments, exposure
Notional amount	90.0	90.0
Gross credit exposure	0.5
Credit Futures
Derivative Financial Instruments, exposure
Notional amount	13.1	10.5
Embedded derivative financial instruments
Derivative Financial Instruments, exposure
Notional amount	$ 9,905.0	$ 9,235.7

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

+ References

No definition available.

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

+ References

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

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- Definition

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

+ References

No definition available.

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- Definition

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

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Represents the amount of cash exchanged under the interest rate derivative contracts.

+ References

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Represents the amount of principal payments made by parties to the interest rate derivative contracts.

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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=56946850&loc=SL5624258-113959

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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 Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=56946850&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=56946850&loc=SL5624258-113959

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