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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)