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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Beginning balance, unpaid claims, gross	$ 4,726	$ 4,359	$ 4,178
Less: Reinsurance and other amounts recoverable	121	115	104
Beginning balance, unpaid claims, net	4,605	4,244	4,074
Incurred claims related to:
Current year	4,341	4,258	3,813
Interest accretion	163	161	163
All other prior years	(4)	93	(91)
Total incurred	4,500	4,512	3,885
Paid claims related to:
Current year	2,724	2,575	2,325
Prior years	1,572	1,560	1,382
Total paid	4,296	4,135	3,707
Acquisitions	0	0	11
Foreign currency	29	(16)	(19)
Ending balance, unpaid claims, net	4,838	4,605	4,244
Add: Reinsurance and other amounts recoverable	137	121	115
Ending balance, unpaid claims, gross	$ 4,975	$ 4,726	$ 4,359