Liabilities for Unpaid Claims and Claims Expenses - Reconciliation to the Liability for Unpaid Claims and Claims Expense (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
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	$ 5,168	$ 4,917
Group Disability And Life Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for unpaid claims and claims expenses, net of reinsurance	4,389
Reinsurance recoverable on unpaid claims	102
Total liability for unpaid claims and claims expenses	4,491	4,342
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	484	384
Other Operations Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liability for unpaid claims and claims expenses	193	$ 191
Long-term Disability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for unpaid claims and claims expenses, net of reinsurance	3,790
Long-term Disability [Member] | Group Disability And Life Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for unpaid claims and claims expenses, net of reinsurance	3,790
Reinsurance recoverable on unpaid claims	94
Other Short-duration Insurance Product Line [Member] | Group Disability And Life Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for unpaid claims and claims expenses, net of reinsurance	599
Reinsurance recoverable on unpaid claims	$ 8