Fair Value (Transfers Between Levels) (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Assets And Liabilities Transferred Between Levels 1 And Levels 2	$ 0	$ 0

Amount of transfers of assets and liabilities measured on a recurring basis between Levels 1 and Levels 2 of the fair value hierarchy.

No definition available.

Name:	met_FairValueAssetsAndLiabilitiesTransferredBetweenLevels1AndLevels2
Namespace Prefix:	met_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration