Derivatives - Credit Risk Assumed through Credit Derivatives (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Credit Derivatives [Line Items]
Notional Amount	$ 40,345	$ 37,756
Fair Value	(461)	(438)
Credit Risk Contract [Member]
Credit Derivatives [Line Items]
Notional Amount	316	480
Fair Value	1	(1)
Offsetting Notional Amount	24	100
Offsetting Fair Value	5	5
Credit Risk Contract [Member] \| Single name credit default swaps \| A Minus \| Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	80	120
Fair Value	$ 2	$ 3
Average Term of Credit Risk Derivatives	5 years	5 years
Offsetting Notional Amount	$ 0	$ 0
Offsetting Fair Value	0	0
Credit Risk Contract [Member] \| Single name credit default swaps \| B \| External Credit Rating, Non Investment Grade [Member]
Credit Derivatives [Line Items]
Notional Amount		43
Fair Value		0
Offsetting Notional Amount		43
Offsetting Fair Value		0
Credit Risk Contract [Member] \| Basket credit default swaps \| A Minus \| Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	13
Fair Value	$ 0
Average Term of Credit Risk Derivatives	5 years
Offsetting Notional Amount	$ 3
Offsetting Fair Value	0
Credit Risk Contract [Member] \| Basket credit default swaps \| BBB Plus \| Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	202	250
Fair Value	$ 4	$ 0
Average Term of Credit Risk Derivatives	5 years	5 years
Offsetting Notional Amount	$ 0	$ 0
Offsetting Fair Value	0	0
Credit Risk Contract [Member] \| Basket credit default swaps \| CCC Plus \| External Credit Rating, Non Investment Grade [Member]
Credit Derivatives [Line Items]
Notional Amount	21
Fair Value	(5)
Offsetting Notional Amount	21
Offsetting Fair Value	$ 5
Credit Risk Contract [Member] \| Basket credit default swaps \| B Plus \| External Credit Rating, Non Investment Grade [Member]
Credit Derivatives [Line Items]
Notional Amount		22
Fair Value		$ 2
Average Term of Credit Risk Derivatives		3 years
Offsetting Notional Amount		$ 22
Offsetting Fair Value		0
Credit Risk Contract [Member] \| Basket credit default swaps \| Standard & Poor's, A Rating [Member] \| Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount		15
Fair Value		$ (1)
Average Term of Credit Risk Derivatives		4 years
Offsetting Notional Amount		$ 5
Offsetting Fair Value		0
Credit Risk Contract [Member] \| Basket credit default swaps \| Standard & Poor's, CCC Rating [Member] \| External Credit Rating, Non Investment Grade [Member]
Credit Derivatives [Line Items]
Notional Amount		30
Fair Value		(5)
Offsetting Notional Amount		30
Offsetting Fair Value		$ 5

Derivative, Nominal Value, Amount Offset Against Collateral, Net

No definition available.

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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

+ Details

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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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4K
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5708775-113959

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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.

+ References

No definition available.

+ Details

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109980867&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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

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