Insurance (Guarantees Related to Universal and Variable Life Contracts) (Details) - Universal and Variable Life Contracts - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Secondary Guarantees
Net Amount at Risk by Product and Guarantee [Line Items]
Total account value (1), (3)	$ 9,254	$ 9,036
Net amount at risk (7)	$ 64,312	$ 66,956
Average attained age of policyholders	57 years	56 years
Paid-Up Guarantees
Net Amount at Risk by Product and Guarantee [Line Items]
Total account value (1), (3)	$ 3,102	$ 3,207
Net amount at risk (7)	$ 15,804	$ 16,615
Average attained age of policyholders	64 years	63 years

Value of the separate and general accounts corresponding to the specified product, guarantee type and subset of the guarantee.

No definition available.

+ Details

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

- Definition

Net amount at risk, representing the guaranteed benefit in excess of the current account balance corresponding to the specified product, guarantee type and subset of the guarantee.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 80
-Section 50
-Paragraph 1
-Subparagraph (d)(3),(5)
-URI http://asc.fasb.org/extlink&oid=6484115&loc=d3e19393-158473

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=6588021&loc=d3e7104-158389

+ Details

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

- Definition

Weighted average attained age of policyholders or contract holders in the specified guarantee type and subset of guarantee class, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 80
-Section 50
-Paragraph 1
-Subparagraph ((d)(5))
-URI http://asc.fasb.org/extlink&oid=6484115&loc=d3e19393-158473

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=6588021&loc=d3e7104-158389

+ Details

Name:	us-gaap_NetAmountAtRiskByProductAndGuaranteeWeightedAverageAttainedAge1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_ProductOrServiceAxis=met_UniversalAndVariableLifeContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteedInsuranceContractTypeOfGuaranteeAxis=met_SecondaryGuaranteesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteedInsuranceContractTypeOfGuaranteeAxis=met_PaidUpGuaranteesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: