Investments (Credit Quality of Agricultural and Residential Mortgage Loans) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Agricultural Mortgage Loans - by Credit Quality Indicator:
Recorded investment in Mortgage Loan	$ 13,014	$ 12,564
% of Total	100.00%	100.00%
Residential Mortgage Loans - by Credit Quality Indicator:
Recorded investment in Mortgage Loan	$ 11,136	$ 10,829
% of Total	100.00%	100.00%
Less than 65%
Agricultural Mortgage Loans - by Credit Quality Indicator:
Recorded investment in Mortgage Loan	$ 12,347	$ 12,023
% of Total	94.90%	95.70%
65% to 75%
Agricultural Mortgage Loans - by Credit Quality Indicator:
Recorded investment in Mortgage Loan	$ 618	$ 436
% of Total	4.70%	3.50%
76% to 80%
Agricultural Mortgage Loans - by Credit Quality Indicator:
Recorded investment in Mortgage Loan	$ 40	$ 17
% of Total	0.30%	0.10%
Greater than 80%
Agricultural Mortgage Loans - by Credit Quality Indicator:
Recorded investment in Mortgage Loan	$ 9	$ 88
% of Total	0.10%	0.70%
Performing
Residential Mortgage Loans - by Credit Quality Indicator:
Recorded investment in Mortgage Loan	$ 10,622	$ 10,448
% of Total	95.40%	96.50%
Nonperforming
Residential Mortgage Loans - by Credit Quality Indicator:
Recorded investment in Mortgage Loan	$ 514	$ 381
% of Total	4.60%	3.50%

Agricultural mortgage loans, by credit quality indicator.

No definition available.

Name:	met_AgriculturalMortgageLoansByCreditQualityIndicatorAbstract
Namespace Prefix:	met_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the percentage of the gross carrying amount of unpaid loans and leases issued to consumers to the total for all consumer loans and leases.

+ References

No definition available.

+ Details

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

- Definition

Reflects the percentage of the gross carrying amount of unpaid loans issued to agricultural businesses to provide financing on existing land or a building, which, pursuant to terms of the underlying debt instrument, are secured at a minimum but possibly exclusively by that real property to the total for all commercial agricultural mortgage loans. Mortgages represent permanent financing, and are issued to replace previously existing mortgages or pay off acquisition, development and construction loans, or to provide cash to the borrower.

+ References

No definition available.

+ Details

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

- Definition

Residential mortgage loans, by credit quality indicator.

+ References

No definition available.

+ Details

Name:	met_ResidentialMortgageLoansByCreditQualityIndicatorAbstract
Namespace Prefix:	met_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount before allowance of consumer mortgage. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes subordinated loans such as home equity lines of credit and loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

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(a)(3),(b))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Amount before allowance of loans issued to agricultural institutions. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans covered under loss sharing agreements.

+ References

Reference 1: 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(a)(1))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5066-111524

+ Details

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

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=met_MortgageLoansByCreditQualityIndicatorRangeOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=met_MortgageLoansByCreditQualityIndicatorRangeTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=met_MortgageLoansByCreditQualityIndicatorRangeThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=met_MortgageLoansByCreditQualityIndicatorRangeFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_PerformingFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_NonperformingFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: