Derivative Instruments - Derivatives - Credit Risk Assumed through Credit Derivatives (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Credit Derivatives [Line Items]
Notional Amount	$ 12,511		$ 12,291
Fair Value	(40)		(61)
Credit Default Swap, Selling Protection [Member]
Credit Derivatives [Line Items]
Notional Amount	829		1,123
Credit derivatives
Credit Derivatives [Line Items]
Fair Value	13		(3)
Offsetting Notional Amount	19		21
Offsetting Fair Value	6		6
Credit derivatives \| Single name credit default swaps \| A Minus \| Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	165
Fair Value	$ 3
Weighted Average Years to Maturity	5 years
Offsetting Notional Amount	$ 0
Offsetting Fair Value	0
Credit derivatives \| Single name credit default swaps \| A \| Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount			169
Fair Value			$ 2
Weighted Average Years to Maturity			4 years
Offsetting Notional Amount			$ 0
Offsetting Fair Value			0
Credit derivatives \| Basket credit default swaps \| A Minus \| Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount			11
Fair Value			$ 0
Weighted Average Years to Maturity			5 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	550		799
Fair Value	$ 10		$ (1)
Weighted Average Years to Maturity	5 years		6 years
Offsetting Notional Amount	$ 0		$ 0
Offsetting Fair Value	0		0
Credit derivatives \| Basket credit default swaps \| B Plus \| Below investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	95		125
Fair Value	$ 6		$ 2
Weighted Average Years to Maturity	5 years		5 years
Offsetting Notional Amount	$ 0		$ 0
Offsetting Fair Value	0		0
Credit derivatives \| Basket credit default swaps \| A \| Investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	1
Fair Value	$ 0
Average Term of Credit Risk Derivatives Less than 1 Year	Less than 1 year
Offsetting Notional Amount	$ 1
Offsetting Fair Value	0
Credit derivatives \| Basket credit default swaps \| CCC \| Below investment grade risk exposure
Credit Derivatives [Line Items]
Notional Amount	18		19
Fair Value	$ (6)		(6)
Average Term of Credit Risk Derivatives Less than 1 Year	Less than 1 year	Less than 1 year
Offsetting Notional Amount	$ 18		19
Offsetting Fair Value	$ 6		$ 6

Average Term of Credit Risk Derivatives Less than 1 Year

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- Definition

Derivative, Nominal Value, Amount Offset Against Collateral, Net

+ References

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

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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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