Derivatives (Credit Risk on Freestanding Derivatives) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Credit Derivatives [Line Items]
Estimated Fair Value of Derivatives in a Net Liability Position (1)	$ 1,156	$ 1,188
Fixed maturity securities AFS
Credit Derivatives [Line Items]
Estimated Fair Value of Collateral Provided:	1,232	1,227
Cash
Credit Derivatives [Line Items]
Estimated Fair Value of Collateral Provided:	0	6
Derivatives Subject to Credit- Contingent Provisions
Credit Derivatives [Line Items]
Estimated Fair Value of Derivatives in a Net Liability Position (1)	1,135	1,148
Derivatives Subject to Credit- Contingent Provisions \| Fixed maturity securities AFS
Credit Derivatives [Line Items]
Estimated Fair Value of Collateral Provided:	1,223	1,218
Derivatives Subject to Credit- Contingent Provisions \| Cash
Credit Derivatives [Line Items]
Estimated Fair Value of Collateral Provided:	0	6
Derivatives Not Subject to Credit- Contingent Provisions
Credit Derivatives [Line Items]
Estimated Fair Value of Derivatives in a Net Liability Position (1)	21	40
Derivatives Not Subject to Credit- Contingent Provisions \| Fixed maturity securities AFS
Credit Derivatives [Line Items]
Estimated Fair Value of Collateral Provided:	9	9
Derivatives Not Subject to Credit- Contingent Provisions \| Cash
Credit Derivatives [Line Items]
Estimated Fair Value of Collateral Provided:	$ 0	$ 0

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624258-113959

+ Details

Name:	us-gaap_CollateralAlreadyPostedAggregateFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_CreditDerivativesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624258-113959

+ Details

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

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_CashMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=met_DerivativesSubjectToCreditContingentProvisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=met_DerivativesNotSubjectToCreditContingentProvisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: