Fair Value Measurements - Level 3 Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Level 3 Financial Assets Carried At Fair Value [Abstract]
Percent Level 3 Fixed Maturities Equity Securities	8.00%
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Percent Level 3 Fixed Maturities Broker Quote	10.00%
Percent Level 3 Securities Transaction Price	10.00%
Other asset and mortgage-backed securities [Member] | Maximum [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity Adjustment Used In Pricing Level 3 Securities Directly Developed By Company	510	410
Weighting Of Credit Spreads Adjustment Used In Pricing Level 3 Securities Directly Developed By Company	4,830	4,540
Other asset and mortgage-backed securities [Member] | Minimum [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity Adjustment Used In Pricing Level 3 Securities Directly Developed By Company	60	60
Weighting Of Credit Spreads Adjustment Used In Pricing Level 3 Securities Directly Developed By Company	110	50
Other asset and mortgage-backed securities [Member] | Weighted Average [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity Adjustment Used In Pricing Level 3 Securities Directly Developed By Company	140	140
Weighting Of Credit Spreads Adjustment Used In Pricing Level 3 Securities Directly Developed By Company	430	410
Corporate and government debt securities [Member] | Maximum [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity Adjustment Used In Pricing Level 3 Securities Directly Developed By Company	675	640
Corporate and government debt securities [Member] | Minimum [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity Adjustment Used In Pricing Level 3 Securities Directly Developed By Company	75	20
Corporate and government debt securities [Member] | Weighted Average [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity Adjustment Used In Pricing Level 3 Securities Directly Developed By Company	170	190
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	$ 16,731	$ 17,816
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	1,288	1,351
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Other asset and mortgage-backed securities [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	632	584
Fair Value Measurements Valuation Techniques	The significant unobservable inputs used to value the following other asset and mortgage-backed securities are liquidity and weighting of credit spreads. An adjustment for liquidity is made as of the measurement date when there is limited trading activity for the security that considers current market conditions, issuer circumstances and complexity of the security structure. An adjustment to weight credit spreads is needed to value a more complex bond structure with multiple underlying collateral with no standard market valuation technique. The weighting of credit spreads is primarily based on the underlying collateral’s characteristics and their proportional cash flows supporting the bond obligations. The resulting wide range of unobservable adjustments in the table below is due to the varying liquidity and quality of the underlying collateral, ranging from high credit quality to below investment grade.
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Other asset and mortgage-backed securities [Member] | Valued Using Pricing Models [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	646	598
Fair Value Measurements Valuation Techniques	Fair values of other asset and mortgage-backed securities, corporate and government fixed maturities are primarily determined using pricing models that incorporate the specific characteristics of each asset and related assumptions including the investment type and structure, credit quality, industry and maturity date in comparison to current market indices, spreads and liquidity of assets with similar characteristics.  For other asset and mortgage-backed securities, inputs and assumptions to pricing may also include collateral attributes and prepayment speeds.
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Corporate and government debt securities [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	331	439
Fair Value Measurements Valuation Techniques	The significant unobservable input used to value the following corporate and government fixed maturities is an adjustment for liquidity. When there is limited trading activity for the security, an adjustment is needed to reflect current market conditions and issuer circumstances.
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Corporate and government debt securities [Member] | Valued Using Pricing Models [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	484	596
Fair Value Measurements Valuation Techniques	Fair values of other asset and mortgage-backed securities, corporate and government fixed maturities are primarily determined using pricing models that incorporate the specific characteristics of each asset and related assumptions including the investment type and structure, credit quality, industry and maturity date in comparison to current market indices, spreads and liquidity of assets with similar characteristics.
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Corporate fixed maturities [Member] | Valued At Transaction Price [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	124	123
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Equity securities [Member] | Valued At Transaction Price [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	$ 34	$ 34