Liabilities for Unpaid Claims and Claims Expenses - Reconciliation to the Liability for Unpaid Claims and Claims Expense (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Group Disability and Life, Global Supplemental Beneftits and Other Operations [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liability for unpaid claims and claims expenses	$ 5,259	$ 5,077
Group Disability And Life Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liability for unpaid claims and claims expenses	4,584	4,442
Global Supplemental Benefits Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liability for unpaid claims and claims expenses	496	453
Other Operations Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liability for unpaid claims and claims expenses	$ 179	$ 182