Investments (Mortgage Loans - Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Mortgage Loans on Real Estate [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Provision for Loan Losses	$ 1	$ 4
Financing Receivable, Significant Purchases	$ 307	762
Percentage of Mortgage Loans Classified as Performing	99.00%		99.00%
Commercial
Mortgage Loans on Real Estate [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Provision for Loan Losses	$ 0	0
Impaired Financing Receivable, Average Recorded Investment	0	12
Agricultural
Mortgage Loans on Real Estate [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Provision for Loan Losses	0	0
Impaired Financing Receivable, Average Recorded Investment	84	25
Estimated fair value of mortgage loans held-for-investment	13,000		$ 13,100
Residential
Mortgage Loans on Real Estate [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Provision for Loan Losses	1	4
Impaired Financing Receivable, Average Recorded Investment	331	$ 253
Estimated fair value of mortgage loans held-for-investment	$ 11,800		$ 11,600

The percentage of all mortgage loans that are classified as performing.

No definition available.

Name:	met_PercentageOfMortgageLoansClassifiedAsPerforming
Namespace Prefix:	met_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of any significant purchases of financing receivables during the reporting period.

+ References

+ Details

Name:	us-gaap_FinancingReceivableSignificantPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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

- Definition

Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_MortgageLoansOnRealEstateLoanTypeAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateLoanTypeAxis=met_AgriculturalPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateLoanTypeAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: