Investments (Net Unrealized Investment Gains Losses) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of net unrealized investment gains (losses) included in accumulated other comprehensive loss
Fixed maturity securities	$ 22,105	$ 30,367
Fixed maturity securities with noncredit OTTI losses in AOCI	(99)	(112)	$ (218)
Total fixed maturity securities	22,006	30,255
Equity securities	372	608
Derivatives	2,145	1,761
Other	287	149
Subtotal	24,810	32,773
Future policy benefits	(183)	(2,886)
DAC and VOBA related to noncredit OTTI losses recognized in AOCI	0	(4)
DAC, VOBA and DSI	(1,456)	(1,946)
Policyholder dividend obligation	(2,309)	(3,155)
Subtotal	(3,948)	(7,991)
Deferred income tax benefit (expense) related to noncredit OTTI losses recognized in AOCI	26	42
Deferred income tax benefit (expense)	(7,210)	(8,556)
Net unrealized investment gains (losses)	13,678	16,268
Net unrealized investment gains (losses) attributable to noncontrolling interests	(44)	(33)
Net unrealized investment gains (losses) attributable to MetLife, Inc.	$ 13,634	$ 16,235