Investments (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2013 item	Mar. 31, 2012	Dec. 31, 2012 item
Total OTTI for AFS securities backed by pools of residential and commercial mortgages	$ 16,000,000	$ 21,000,000
OTTI for AFS securities backed by pools of residential and commercial mortgages recognized in OCI	(33,000,000)	(21,000,000)
OTTI for AFS securities backed by pools of residential and commercial mortgages recognized in net income (loss)	17,000,000	42,000,000
Decrease in gross AFS securities unrealized losses	27,000,000
Number of partnerships in alternative investment portfolio	100		98
Alternative investments as a percentage of overall invested assets	1.00%		1.00%
Investment commitments	888,000,000
Investment commitments for limited partnerships	369,000,000
Investment commitments for private placements	372,000,000
Investment commitments for mortgage loans on real estate	147,000,000
California And Texas [Member]
Largest mortgage loan concentration in geographic region	32.00%		32.00%
Federal Home Loan Mortgage Corporation
Fair value	3,400,000,000		3,800,000,000
Concentration risk, percentage	3.00%		4.00%
Fannie Mae
Fair value	2,100,000,000		2,200,000,000
Concentration risk, percentage	2.00%		2.00%
Electric Industry [Member]
Fair value	8,800,000,000		8,700,000,000
Concentration risk, percentage	9.00%		9.00%
Investment in CMO industry
Fair value	5,000,000,000		5,000,000,000
Concentration risk, percentage	5.00%		5.00%
Corporate Bonds [Member]
Percentage of fair value rated as investment grade	96.00%		96.00%
Amortized cost of portfolio rated below investment grade	2,900,000		3,000,000
Fair value of portfolio rated below investment grade	2,900,000		2,900,000
OTTI securities recognized in net income (loss), Recovery percentage of amortized cost	98.00%	92.00%
CDOs [Member]
Percentage of fair value rated as investment grade	93.00%		93.00%
Amortized cost of portfolio rated below investment grade	19,000,000		21,000,000
Fair value of portfolio rated below investment grade	$ 12,000,000		13,000,000
OTTI securities recognized in net income (loss), Recovery percentage of amortized cost	94.00%	0.00%
MBS [Member]
Projected default rate on existing delinquencies on MBS (low end of range)	10.00%
Projected default rate on existing delinquencies on MBS (high end of range)	100.00%
Severity of second lien loans	100.00%
Severity of first lien loans	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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No definition available.

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- Definition

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.

+ References

No definition available.

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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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No definition available.

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Recovery as a percentage of amortized cost for securities with other than temporary impairment recorded in net income (loss).

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Fair value of portfolio rated below investment grade.

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- Definition

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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No definition available.

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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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- Definition

Total other-than-temporary impairment for available-for-sale securities backed by pools of residential and commercial mortgages recognized during the reporting period.

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No definition available.

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Other-than-temporary impairment for available-for-sale securities backed by pools of residential and commercial mortgages relating to credit losses and recognized in net income (loss) during the reporting period.

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No definition available.

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Other-than-temporary impairment for available-for-sale securities backed by pools of residential and commercial mortgages relating to factors other than credit losses and recognized in other comprehensive income during the reporting period.

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No definition available.

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- Definition

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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No definition available.

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- Definition

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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No definition available.

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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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- Definition

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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