Liabilities for Unpaid Claims and Claims Expenses - Liability Balance Details (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Group Disability and Life, Global Supplemental Beneftits and Other Operations [Member]
Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Total liability for unpaid claims and claims expenses	$ 5,168	$ 4,917
Group Disability And Life Segment [Member]
Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Total liability for unpaid claims and claims expenses	4,491	4,342
Global Supplemental Benefits Segment [Member]
Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Total liability for unpaid claims and claims expenses	484	384
Other Operations Segment [Member]
Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Total liability for unpaid claims and claims expenses	$ 193	$ 191