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	9 Months Ended
	Sep. 30, 2018	Sep. 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	3,590	3,290
Interest accretion	115	119
All other prior years	(130)	(59)
Total incurred	3,575	3,350
Paid claims related to:
Current year	2,060	1,880
Prior years	1,390	1,327
Total paid	3,450	3,207
Foreign currency	(16)	13
Ending balance, unpaid claims, net	4,947	4,761
Add: Reinsurance and other amounts recoverable	133	134
Ending balance, unpaid claims, gross	$ 5,080	$ 4,895