Investments (Narrative) (Details) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($) security item	Dec. 31, 2018 USD ($) security item
Investment [Line Items]
Decrease in gross AFS securities unrealized losses	$ (1,400)
Number of partnerships in alternative investment portfolio \| item	243	234
Alternative investments as a percentage of overall invested assets	2.00%	1.00%
Fair value of collateral received that we are permitted to sell or re-pledge	$ 548
Securities that have been re-pledged	536
Investment commitments	2,000
Investment commitments for limited partnerships	823
Investment commitments for mortgage loans on real estate	661
Investment commitments for private placements	$ 528
Commercial [Member]
Investment [Line Items]
Number of impaired loans \| security	0	0
Residential [Member]
Investment [Line Items]
Number of impaired loans \| security	0	0
Impaired financing receivable, allowance	$ 1
Residential [Member] \| Maximum [Member]
Investment [Line Items]
Impaired financing receivable, allowance		$ 1
Federal Home Loan Mortgage Corporation [Member] \| Invested Assets [Member]
Investment [Line Items]
Fair value	$ 1,300	$ 1,400
Concentration risk, percentage	1.00%	1.00%
Fannie Mae [Member] \| Invested Assets [Member]
Investment [Line Items]
Fair value	$ 1,300	$ 1,200
Concentration risk, percentage	1.00%	1.00%
Financial Service [Member] \| Invested Assets [Member]
Investment [Line Items]
Fair value	$ 17,100	$ 16,000
Concentration risk, percentage	14.00%	14.00%
Consumer Non-Cyclical Industry [Member] \| Invested Assets [Member]
Investment [Line Items]
Fair value	$ 14,300	$ 13,800
Concentration risk, percentage	12.00%	12.00%
Mortgage Loans On Real Estate [Member] \| Geographic Concentration Risk [Member] \| Commercial [Member] \| California [Member]
Investment [Line Items]
Concentration risk, percentage	23.00%	23.00%
Mortgage Loans On Real Estate [Member] \| Geographic Concentration Risk [Member] \| Commercial [Member] \| Texas [Member]
Investment [Line Items]
Concentration risk, percentage	12.00%	12.00%
Mortgage Loans On Real Estate [Member] \| Geographic Concentration Risk [Member] \| Residential [Member] \| California [Member]
Investment [Line Items]
Concentration risk, percentage	30.00%	34.00%
Mortgage Loans On Real Estate [Member] \| Geographic Concentration Risk [Member] \| Residential [Member] \| Florida [Member]
Investment [Line Items]
Concentration risk, percentage	19.00%	19.00%
Corporate Bonds [Member]
Investment [Line Items]
Percentage of fair value rated as investment grade	96.00%	96.00%
Amortized cost of portfolio rated below investment grade	$ 3,200	$ 3,100
Fair value of portfolio rated below investment grade	$ 3,100	$ 2,900
MBS [Member]
Investment [Line Items]
Severity of second lien loans	100.00%	100.00%
Severity of first lien loans	30.00%	30.00%

Alternative investments as a percentage of overall invested assets.

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Amortized cost of portfolio rated below investment grade.

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This item represents the change in gross unrealized holding gain or loss on available-for-sale securities that has been included in accumulated other comprehensive income, a separate component of shareholders' equity, during the period.

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Percentage of the fair value of portfolio that was rated investment grade as of balance sheet date.

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Fair value of portfolio rated below investment grade.

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Impaired financing receivable, number of impaired loans

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This item represents total investment commitments as of the balance sheet date for fixed maturity available-for-sale securities, limited partnerships, real estate and mortgage loans on real estate.

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Number of partnerships represented in alternative investment portfolio.

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This item represents the loss severity as a percentage of first lien loans that was considered as one of the potential factors in reaching a conclusion that the available-for-sale investments are other than temporarily impaired and the amount of such impairment determined to be related to credit loss recognized in earnings.

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This item represents the loss severity as a percentage of defaulted second lien loans that was considered as one of the potential factors in reaching a conclusion that the available-for-sale investments are other than temporarily impaired and the amount of such impairment determined to be related to credit loss recognized in earnings.

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Line items represent reporting 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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Fair value of collateral permitted, by contact or custom, to be sold or re-pledged.

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Fair value of the collateral, or portion thereof, that has been re-pledged, delivered, or otherwise used.

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Amount of allowance for credit losses related to recorded investment.

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