Level 4~Derivatives - Credit Risk Assumed through Credit Derivatives (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Credit Derivatives [Line Items]
Notional Amount	$ 12,875	$ 12,730
Fair Value	(74)	(90)
Credit Default Swap, Selling Protection [Member]
Credit Derivatives [Line Items]
Notional Amount	1,461	1,346
Credit Risk Derivatives, at Fair Value, Net	7	0
Credit derivatives \| Single name credit default swaps \| A Minus \| Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	170	130
Fair Value	$ 4	$ 3
Weighted Average Years to Maturity	5 years	5 years
Offsetting Notional Amount	$ 0	$ 0
Offsetting Fair Value	0	0
Credit derivatives \| Single name credit default swaps \| B \| Below investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount		9
Fair Value		0
Offsetting Notional Amount		9
Offsetting Fair Value		0
Credit derivatives \| Basket credit default swaps \| A Minus \| Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	12
Fair Value	$ (1)
Weighted Average Years to Maturity	6 years
Offsetting Notional Amount	$ 2
Offsetting Fair Value	0
Credit derivatives \| Basket credit default swaps \| BBB Plus \| Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	1,252	1,137
Fair Value	$ 10	$ 2
Weighted Average Years to Maturity	4 years	3 years
Offsetting Notional Amount	$ 453	$ 454
Offsetting Fair Value	(1)	(2)
Credit derivatives \| Basket credit default swaps \| B Plus \| Below investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount		27
Fair Value		$ 2
Weighted Average Years to Maturity		3 years
Offsetting Notional Amount		$ 27
Offsetting Fair Value		0
Credit derivatives \| Basket credit default swaps \| A \| Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount		13
Fair Value		$ (1)
Weighted Average Years to Maturity		5 years
Offsetting Notional Amount		$ 3
Offsetting Fair Value		0
Credit derivatives \| Basket credit default swaps \| CCC \| Below investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	27	30
Fair Value	(6)	(6)
Offsetting Notional Amount	27	30
Offsetting Fair Value	6	7
Credit derivatives in offsetting positions \| Credit Default Swap, Selling Protection [Member]
Credit Derivatives [Line Items]
Notional Amount	482	523
Credit Risk Derivatives, at Fair Value, Net	$ 5	$ 5

Derivative, Nominal Value, Amount Offset Against Collateral, Net

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- Definition

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

+ References

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

Average period covered by credit risk derivative contracts, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ 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.

+ References

No definition available.

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- Definition

Fair value of credit risk derivative asset after deduction of credit risk derivative liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4K
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5708775-113959

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611

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- Definition

The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that in accordance with the entity's accounting policy were offset against collateral under a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959

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- Definition

Fair value of the assets less the liabilities of a derivative or group of derivatives.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

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-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611

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