Investments (Variable Interest Entities - Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Tax credits guaranteed by third parties that reduce maximum exposure to loss related to other invested assets	$ 101	$ 117

This caption discloses the amount of tax credits guaranteed by third parties that reduces the Company's maximum exposure to loss related to other invested assets plus any unfunded commitments.

No definition available.

Name:	met_TaxCreditsGuaranteedByThirdPartiesAmountThatReducesMaximumExposureToLossRelatedToOtherInvestedAssets
Namespace Prefix:	met_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant