Investments (Net Unrealized Investment Gains Losses) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of net unrealized investment gains (losses) included in accumulated other comprehensive loss
Fixed maturity securities	$ 36,444	$ 18,164
Fixed maturity securities with noncredit OTTI losses included in AOCI	(13)	(76)	$ (112)
Total fixed maturity securities	36,431	18,088
Equity securities	579	422
Derivatives	3,648	2,350
Other	397	287
Subtotal	41,055	21,147
Future policy benefits	(5,571)	(163)
DAC and VOBA related to noncredit OTTI losses recognized in AOCI	(2)	0
DAC, VOBA and DSI	(2,390)	(1,273)
Policyholder dividend obligation	(3,352)	(1,783)
Subtotal	(11,315)	(3,219)
Deferred income tax benefit (expense) related to noncredit OTTI losses recognized in AOCI	7	27
Deferred income tax benefit (expense)	(10,078)	(6,151)
Net unrealized investment gains (losses)	19,669	11,804
Net unrealized investment gains (losses) attributable to noncontrolling interests	(12)	(31)
Net unrealized investment gains (losses) attributable to MetLife, Inc.	$ 19,657	$ 11,773