Junior Subordinated Debt Securities (Junior Subordinated Debt Securities) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Face Value	$ 3,200	$ 3,200
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	(88)	(87)
Junior Subordinated Notes	$ 3,150	3,147
MetLife Inc $500M Maturing 2069 [Member]
Debt Instrument [Line Items]
Issue Date	Jul. 08, 2009
Face Value	$ 500	500
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ (7)	(7)
Debt Instrument, Interest Rate, Stated Percentage	10.75%
Debt Instrument, Interest Rate Terms	LIBOR + 7.548%
Final Maturity	Aug. 01, 2069
Junior Subordinated Notes	$ 493	493
Debt Conversion, Converted Instrument, Expiration or Due Date, Month and Year	2039-08
Senior notes
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ (81)	(79)
Debt Instrument, Interest Rate, Stated Percentage	5.70%
Final Maturity	Jun. 30, 2035
MetLife Capital Trust X $750M Maturing 2068 [Member]
Debt Instrument [Line Items]
Issue Date	Apr. 08, 2008
Face Value	$ 750	750
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ (10)	(11)
Debt Instrument, Interest Rate, Stated Percentage	9.25%
Debt Instrument, Interest Rate Terms	LIBOR + 5.540%
Final Maturity	Apr. 08, 2068
Junior Subordinated Notes	$ 740	739
Debt Conversion, Converted Instrument, Expiration or Due Date, Month and Year	2038-04
MetLife Capital Trust IV $700M Maturing 2067 [Member]
Debt Instrument [Line Items]
Issue Date	Dec. 15, 2007
Face Value	$ 700	700
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ (15)	(16)
Debt Instrument, Interest Rate, Stated Percentage	7.875%
Debt Instrument, Interest Rate Terms	LIBOR + 3.960%
Final Maturity	Dec. 15, 2067
Junior Subordinated Notes	$ 685	684
Debt Conversion, Converted Instrument, Expiration or Due Date, Month and Year	2037-12
MetLife Inc $1,250M Maturing 2066 [Member]
Debt Instrument [Line Items]
Issue Date	Dec. 19, 2006
Face Value	$ 1,250	1,250
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ (18)	(19)
Debt Instrument, Interest Rate, Stated Percentage	6.40%
Debt Instrument, Interest Rate Terms	LIBOR + 2.205%
Final Maturity	Dec. 15, 2066
Junior Subordinated Notes	$ 1,232	1,231
Debt Conversion, Converted Instrument, Expiration or Due Date, Month and Year	2036-12
Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ (50)	$ (53)

The expiration, mandatory redemption, or due date, month and year (YYYY-MM), of the financial instrument issued in exchange for the original debt being converted in a noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586

+ Details

Name:	us-gaap_DebtConversionConvertedInstrumentExpirationOrDueDateMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Description of the interest rate as being fixed or variable, and, if variable, identification of the index or rate on which the interest rate is based and the number of points or percentage added to that index or rate to set the rate, and other pertinent information, such as frequency of rate resets.

+ References

+ Details

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

- Definition

Date the debt instrument was issued, in CCYY-MM-DD format.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentIssuanceDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of unamortized debt discount (premium) and debt issuance costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28555-108399

+ Details

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

- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Junior Subordinated Notes, which have a lower priority than senior instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16)(a))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=met_JuniorSubordinatedDebtInstrumentOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=met_JuniorSubordinatedDebtInstrumentTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=met_JuniorSubordinatedDebtInstrumentThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=met_JuniorSubordinatedDebtInstrumentFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_JuniorSubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: