Investments (Narrative) (Details) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2016 USD ($) item	Dec. 31, 2015 USD ($) item
Increase (decrease) in gross AFS securities unrealized losses	$ (1,600)
Number of partnerships in alternative investment portfolio \| item	198	190
Alternative investments as a percentage of overall invested assets	1.00%	1.00%
Fair value of collateral received that we are permitted to sell or re-pledge	$ 175
Investment commitments	1,400
Investment commitments for limited partnerships	727
Investment commitments for mortgage loans on real estate	503
Investment commitments for private placements	$ 186
California [Member]
Largest mortgage loan concentration in geographic region	20.00%	21.00%
Texas [Member]
Largest mortgage loan concentration in geographic region	11.00%	10.00%
Federal Home Loan Mortgage Corporation
Fair value	$ 1,700	$ 1,800
Concentration risk, percentage	2.00%	2.00%
Fannie Mae
Fair value	$ 1,200	$ 1,200
Concentration risk, percentage	1.00%	1.00%
Utilities Industry [Member]
Fair value	$ 14,000	$ 12,800
Concentration risk, percentage	13.00%	12.00%
Consumer Non-Cyclical Industry [Member]
Fair value	$ 14,400	$ 12,000
Concentration risk, percentage	13.00%	12.00%
Corporate bonds [Member]
Percentage of fair value rated as investment grade	95.00%	96.00%
Amortized cost of portfolio rated below investment grade	$ 4,000	$ 3,600
Fair value of portfolio rated below investment grade	$ 3,800	$ 3,300
ABS [Member]
Percentage of fair value rated as investment grade	96.00%	96.00%
Amortized cost of portfolio rated below investment grade	$ 103	$ 107
Fair value of portfolio rated below investment grade	$ 88	$ 92
MBS [Member]
Projected default rate on existing delinquencies on MBS (low end of range)	10.00%	10.00%
Projected default rate on existing delinquencies on MBS (high end of range)	100.00%	100.00%
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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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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This item represents investment commitments as of the balance sheet date for limited partnerships.

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This item represents investment commitments as of the balance sheet date for private placements.

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This item represents investment commitments as of the balance sheet date for mortgage loans.

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Largest concentration percentage of mortgage loans on commercial real estate throughout the U.S.

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Number of partnerships represented in alternative investment portfolio.

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This item represents the default rate (as a percentage) of the underlying assets in the security 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 (high end of range).

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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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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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This item represents disclosure of all significant concentrations of credit risk or market risk arising from the subject financial instrument (as defined), whether from an individual counterparty or groups of counterparties.

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

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