Fair Value (Assets and liabilities measured at fair value on a recurring basis) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	$ 42,406	$ 41,442
Corporate and other bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	18,970	17,760
States, municipalities and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	13,240	13,174
Residential mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	5,073	5,072
Commercial mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	2,040	2,197
Other asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	1,025	921
Total asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	8,138	8,190
U.S. Treasury and obligations of government-sponsored enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	93	67
Foreign Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	445	346
Redeemable preferred stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	19	35
Fixed maturity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	40,905	39,572
Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	110	197
Other invested assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	5	17
Short term investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	1,328	1,582
Life settlement contracts, included in Other assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	58	74
Embedded Derivative Financial Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of liabilities, measured on a recurring basis	(3)	(5)
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	678	726
Fair value of liabilities, measured on a recurring basis	0	0
Level 1 [Member] | Corporate and other bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 1 [Member] | States, municipalities and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 1 [Member] | Residential mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 1 [Member] | Commercial mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 1 [Member] | Other asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 1 [Member] | Total asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 1 [Member] | U.S. Treasury and obligations of government-sponsored enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	93	66
Level 1 [Member] | Foreign Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 1 [Member] | Redeemable preferred stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	19	35
Level 1 [Member] | Fixed maturity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	112	101
Level 1 [Member] | Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	91	177
Level 1 [Member] | Other invested assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 1 [Member] | Short term investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	475	448
Level 1 [Member] | Life settlement contracts, included in Other assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 1 [Member] | Embedded Derivative Financial Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of liabilities, measured on a recurring basis	0	0
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	41,321	40,243
Fair value of liabilities, measured on a recurring basis	(3)	(5)
Level 2 [Member] | Corporate and other bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	18,840	17,592
Level 2 [Member] | States, municipalities and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	13,239	13,172
Level 2 [Member] | Residential mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	4,944	4,938
Level 2 [Member] | Commercial mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	2,027	2,175
Level 2 [Member] | Other asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	968	868
Level 2 [Member] | Total asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	7,939	7,981
Level 2 [Member] | U.S. Treasury and obligations of government-sponsored enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	1
Level 2 [Member] | Foreign Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	445	346
Level 2 [Member] | Redeemable preferred stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 2 [Member] | Fixed maturity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	40,463	39,092
Level 2 [Member] | Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 2 [Member] | Other invested assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	5	17
Level 2 [Member] | Short term investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	853	1,134
Level 2 [Member] | Life settlement contracts, included in Other assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 2 [Member] | Embedded Derivative Financial Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of liabilities, measured on a recurring basis	(3)	(5)
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	407	473
Fair value of liabilities, measured on a recurring basis	0	0
Level 3 [Member] | Corporate and other bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	130	168
Level 3 [Member] | States, municipalities and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	1	2
Level 3 [Member] | Residential mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	129	134
Level 3 [Member] | Commercial mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	13	22
Level 3 [Member] | Other asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	57	53
Level 3 [Member] | Total asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	199	209
Level 3 [Member] | U.S. Treasury and obligations of government-sponsored enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 3 [Member] | Foreign Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 3 [Member] | Redeemable preferred stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 3 [Member] | Fixed maturity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	330	379
Level 3 [Member] | Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	19	20
Level 3 [Member] | Other invested assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 3 [Member] | Short term investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 3 [Member] | Life settlement contracts, included in Other assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	58	74
Level 3 [Member] | Embedded Derivative Financial Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of liabilities, measured on a recurring basis	$ 0	$ 0