Additional Information - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for reinsurance recoverables [Line Items]
Total rental expense	$ 67	$ 70	$ 65
Operating lease future minimum payments due in next year	66
Operating lease future minimum payments due in year two	60
Operating lease future minimum payments due in year three	53
Operating lease future minimum payments due in year four	45
Operating lease future minimum payments due in year five	37
Operating lease future minimum payments due after year five	128
Commitments to fund limited partnerships	377
Retirement and employee savings plan expense	43	34	30
Allowance for reinsurance recoverables
Allowance for reinsurance recoverables [Line Items]
Aggregate allowance for losses on reinsurance recoverables	22	22
Net expense reduction against allowance for losses on reinsurance recoverables		$ (3)
Charges for possible losses on reinsurance recoverables			4
Reinsurance recoverable write-offs			$ 6
Maximum | Allowance for reinsurance recoverables
Allowance for reinsurance recoverables [Line Items]
Net expense reduction against allowance for losses on reinsurance recoverables	$ (1)