Liabilities for Unpaid Claims and Claims Expenses - Activity in Liabilities for Unpaid Claims and Claims Expenses (Details) - Group Disability and Life and Global Supplemental Benefits [Member] - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Beginning balance, unpaid claims, gross	$ 4,975	$ 4,726
Less: Reinsurance and other amounts recoverable	137	121
Beginning balance, unpaid claims, net	4,838	4,605
Incurred claims related to:
Current year	2,382	2,224
Interest accretion	77	83
All other prior years	(66)	(71)
Total incurred	2,393	2,236
Paid claims related to:
Current year	1,204	1,074
Prior years	1,101	1,058
Total paid	2,305	2,132
Foreign currency	(14)	13
Ending balance, unpaid claims, net	4,912	4,722
Add: Reinsurance and other amounts recoverable	141	130
Ending balance, unpaid claims, gross	$ 5,053	$ 4,852