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Additional Information - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Allowance for reinsurance recoverables [Line Items] |  |  |
Total rental expense | $ 69 | $ 67 | $ 70
Operating lease future minimum payments due in next year | 63 |  |
Operating lease future minimum payments due in year two | 57 |  |
Operating lease future minimum payments due in year three | 50 |  |
Operating lease future minimum payments due in year four | 42 |  |
Operating lease future minimum payments due in year five | 34 |  |
Operating lease future minimum payments due after year five | 100 |  |
Commitments to fund limited partnerships | 484 |  |
Retirement and employee savings plan expense | 45 | 43 | 34
Allowance for reinsurance recoverables |  |  |
Allowance for reinsurance recoverables [Line Items] |  |  |
Aggregate allowance for losses on reinsurance recoverables | 20 | 22 |
Net expense reduction against the allowance for losses on reinsurance recoverables |  |  | $ (3)
Reinsurance recoverable write-offs | 2 |  |
Maximum | Allowance for reinsurance recoverables |  |  |
Allowance for reinsurance recoverables [Line Items] |  |  |
Net expense reduction against the allowance for losses on reinsurance recoverables | $ (1) | $ (1) |

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