Insurance (Obligations Under Funding Agreements - Liability and Collateral) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Outstanding Funding Agreements To Certain SPEs	$ 39,900	$ 34,600
Funding Agreements Farmer Mac
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Outstanding Funding Agreements To Certain SPEs	2,375	2,550
Invested Assets Pledged As Collateral	2,450	2,670
Federal Home Loan Bank of New York
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank amount of advances by branch for funding agreements	16,200	14,445
Collateral pledged relating to obligations under funding agreements	18,539	16,570
Federal Home Loan Bank of Des Moines
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank amount of advances by branch for funding agreements	50	100
Collateral pledged relating to obligations under funding agreements	72	141
Federal Home Loan Bank of Pittsburgh
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Federal Home Loan Bank amount of advances by branch for funding agreements	0	775
Collateral pledged relating to obligations under funding agreements	$ 0	$ 895

Invested assets pledged as collateral.

No definition available.

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

- Definition

Amount of funding agreements outstanding to certain SPEs that have issued debt securities for which payment of interest and principal is secured by such funding agreements.

+ References

No definition available.

+ Details

Name:	met_OutstandingFundingAgreementsToCertainSpecialPurposeEntities
Namespace Prefix:	met_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of advances from the Federal Home Loan Bank (FHLB).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankAmountOfAdvancesByBranch
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the collateral pledged in support of federal home loan bank advances outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	met_FixedAndFloatingRateFundingAgreementsByTypeAxis=met_FundingAgreementsFarmerMacMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_FederalHomeLoanBankAdvancesBranchOfFHLBBankAxis=srt_FederalHomeLoanBankOfNewYorkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_FederalHomeLoanBankAdvancesBranchOfFHLBBankAxis=srt_FederalHomeLoanBankOfDesMoinesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_FederalHomeLoanBankAdvancesBranchOfFHLBBankAxis=srt_FederalHomeLoanBankOfPittsburghMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: