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Fair Value (Assets and liabilities measured at fair value on a recurring basis) (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | $ 43,536 | $ 42,406
Fair value of liabilities, measured on a recurring basis | 3 | (3)
Corporate and other bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 19,374 | 19,527
States, municipalities and political subdivisions |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 14,027 | 13,240
Total asset-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 8,086 | 8,138
Fixed maturity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 41,487 | 40,905
Equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 695 | 110
Short term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 1,354 | 1,333
Life settlement contracts, included in Other assets |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis |  | 58
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 615 | 678
Fair value of liabilities, measured on a recurring basis | 0 | 0
Level 1 | Corporate and other bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 128 | 112
Level 1 | States, municipalities and political subdivisions |  |
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 | Total asset-backed |  |
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 | Fixed maturity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 128 | 112
Level 1 | Equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 91 | 91
Level 1 | Short term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 396 | 475
Level 1 | Life settlement contracts, included in Other assets |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis |  | 0
Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 42,467 | 41,321
Fair value of liabilities, measured on a recurring basis | 3 | (3)
Level 2 | Corporate and other bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 19,148 | 19,285
Level 2 | States, municipalities and political subdivisions |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 14,026 | 13,239
Level 2 | Total asset-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 7,751 | 7,939
Level 2 | Fixed maturity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 40,925 | 40,463
Level 2 | Equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 584 | 0
Level 2 | Short term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 958 | 858
Level 2 | Life settlement contracts, included in Other assets |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis |  | 0
Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 454 | 407
Fair value of liabilities, measured on a recurring basis | 0 | 0
Level 3 | Corporate and other bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 98 | 130
Level 3 | States, municipalities and political subdivisions |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 1 | 1
Level 3 | Total asset-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 335 | 199
Level 3 | Fixed maturity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 434 | 330
Level 3 | Equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 20 | 19
Level 3 | Short term investments |  |
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 | Life settlement contracts, included in Other assets |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis |  | $ 58

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