Liabilities for Unpaid Claims and Claims Expenses - Liability Balance Details (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
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	$ 5,215	$ 5,006
Group Disability And Life Segment [Member]
Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Total liability for unpaid claims and claims expenses	4,549	4,384
Global Supplemental Benefits Segment [Member]
Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Total liability for unpaid claims and claims expenses	492	425
Other Operations Segment [Member]
Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Total liability for unpaid claims and claims expenses	$ 174	$ 197