Derivatives (Fair Value Hedges) (Details) - Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Fixed maturity securities AFS
Derivative Instruments, Gain (Loss) [Line Items]
Hedged Asset, Discontinued Fair Value Hedge, Cumulative Increase (Decrease)	$ (1)	$ (1)
Debt Instruments, Carrying Amount	2,635	2,736
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	(1)	(1)
Mortgages [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Debt Instruments, Carrying Amount	816	1,159
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	21	2
Future policy benefits [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Debt Instruments, Carrying Amount	(5,747)	(4,475)
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	$ (1,660)	$ (908)

Debt Instruments, Carrying Amount

No definition available.

Name:	met_DebtInstrumentsCarryingAmount
Namespace Prefix:	met_
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_DerivativeInstrumentsGainLossLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk, remaining after discontinued hedge.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109999712-113959

+ Details

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

- Definition

Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109999712-113959

+ Details

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

- Details

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

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=met_FuturepolicybenefitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: