Derivative Financial Instruments - Notional Amounts and Credit Exposure (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Derivative Financial Instruments, exposure
Cash and securities posted under collateral arrangements associated with derivative credit support agreements and Futures Commission Merchant agreements	$ 195.0	$ 322.4
Aggregate fair value of all derivative instruments with credit-risk-related contingent features that were in a liability position	280.4	454.7
Collateral and initial margin posted supporting derivatives with credit-risk-related contingent features that were in a liability position	195.0	322.4
Additional collateral required to be posted if derivative credit-risk-related contingent features were triggered	31.8
Cash collateral received associated with derivative credit support annex agreements and Futures Commission Merchant agreements	124.7	576.3
Notional amount	40,916.6	46,370.8
Gross credit exposure	311.4	908.9
Less: collateral received	159.7	586.8
Net credit exposure	151.7	322.1
Interest rate swaps
Derivative Financial Instruments, exposure
Cash exchanged under contract	0.0
Principal payments made under contract	0.0
Notional amount	23,543.4	23,520.4
Gross credit exposure	163.4	733.1
Interest rate options
Derivative Financial Instruments, exposure
Notional amount	657.0	4,950.5
Gross credit exposure	19.8	27.3
Interest rate futures
Derivative Financial Instruments, exposure
Notional amount	236.5	96.0
Swaptions
Derivative Financial Instruments, exposure
Notional amount	14.0	77.0
Currency swaps
Derivative Financial Instruments, exposure
Notional amount	888.6	1,552.0
Gross credit exposure	79.1	106.2
Currency forwards
Derivative Financial Instruments, exposure
Notional amount	915.5	851.3
Gross credit exposure	24.3	6.4
Currency options
Derivative Financial Instruments, exposure
Notional amount	583.6
Gross credit exposure	1.6
Equity options
Derivative Financial Instruments, exposure
Notional amount	3,649.4	3,505.8
Gross credit exposure	18.2	28.2
Equity futures
Derivative Financial Instruments, exposure
Notional amount	357.8	545.1
Credit default swaps
Derivative Financial Instruments, exposure
Notional amount	668.5	961.3
Gross credit exposure	5.0	7.0
Total return swaps
Derivative Financial Instruments, exposure
Notional amount		90.0
Gross credit exposure		0.7
Credit Futures
Derivative Financial Instruments, exposure
Notional amount		11.9
Embedded derivative financial instruments
Derivative Financial Instruments, exposure
Notional amount	$ 9,402.3	$ 10,209.5

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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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=84234895&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=84234895&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=84234895&loc=SL5624258-113959

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