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	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	1,230	1,148
Interest accretion	38	43
All other prior years	(46)	(64)
Total incurred	1,222	1,127
Paid claims related to:
Current year	430	371
Prior years	728	691
Total paid	1,158	1,062
Foreign currency	2	16
Ending balance, unpaid claims, net	4,904	4,686
Add: Reinsurance and other amounts recoverable	137	123
Ending balance, unpaid claims, gross	$ 5,041	$ 4,809