Investments (Fixed Maturity Securities Available-For-Sale by Sector) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $3,400 and $3,062, respectively)	$ 310,811	$ 297,655
Allowance for Credit Loss for Debt Securities	(81)	0
Estimated Fair Value of Fixed Maturity Securities AFS	354,809	327,820
Fixed Maturity Securities
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gain	45,519	31,812
Gross Unrealized Temporary Loss	1,440	1,680
Gross Unrealized OTTI		(33)
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $3,400 and $3,062, respectively)	79,788	79,115
Allowance for Credit Loss for Debt Securities	(44)	0
Gross Unrealized Gain	13,924	8,943
Gross Unrealized Temporary Loss	252	305
Gross Unrealized OTTI		0
Estimated Fair Value of Fixed Maturity Securities AFS	93,416	87,753
Foreign government
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $3,400 and $3,062, respectively)	63,243	58,840
Allowance for Credit Loss for Debt Securities	(21)	0
Gross Unrealized Gain	8,883	8,710
Gross Unrealized Temporary Loss	406	321
Gross Unrealized OTTI		0
Estimated Fair Value of Fixed Maturity Securities AFS	71,699	67,229
Foreign corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $3,400 and $3,062, respectively)	60,995	59,342
Allowance for Credit Loss for Debt Securities	(16)	0
Gross Unrealized Gain	8,897	5,540
Gross Unrealized Temporary Loss	468	717
Gross Unrealized OTTI		0
Estimated Fair Value of Fixed Maturity Securities AFS	69,408	64,165
U.S. government and agency
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $3,400 and $3,062, respectively)	39,094	37,586
Allowance for Credit Loss for Debt Securities	0
Gross Unrealized Gain	8,095	4,604
Gross Unrealized Temporary Loss	89	106
Gross Unrealized OTTI		0
Estimated Fair Value of Fixed Maturity Securities AFS	47,100	42,084
RMBS
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $3,400 and $3,062, respectively)	28,415	27,051
Allowance for Credit Loss for Debt Securities	0	0
Gross Unrealized Gain	2,062	1,535
Gross Unrealized Temporary Loss	42	72
Gross Unrealized OTTI		(33)
Estimated Fair Value of Fixed Maturity Securities AFS	30,435	28,547
ABS
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $3,400 and $3,062, respectively)	16,963	14,547
Allowance for Credit Loss for Debt Securities	0
Gross Unrealized Gain	231	83
Gross Unrealized Temporary Loss	75	88
Gross Unrealized OTTI		0
Estimated Fair Value of Fixed Maturity Securities AFS	17,119	14,542
Municipals
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $3,400 and $3,062, respectively)	10,982	11,081
Allowance for Credit Loss for Debt Securities	0
Gross Unrealized Gain	2,746	2,001
Gross Unrealized Temporary Loss	6	29
Gross Unrealized OTTI		0
Estimated Fair Value of Fixed Maturity Securities AFS	13,722	13,053
CMBS
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $3,400 and $3,062, respectively)	11,331	10,093
Allowance for Credit Loss for Debt Securities	0
Gross Unrealized Gain	681	396
Gross Unrealized Temporary Loss	102	42
Gross Unrealized OTTI		0
Estimated Fair Value of Fixed Maturity Securities AFS	$ 11,910	$ 10,447

Other-than-temporary impairments on fixed maturity securities transferred to other comprehensive income loss.

No definition available.

Name:	met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
Namespace Prefix:	met_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

+ Details

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

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

No definition available.

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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

- Definition

Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DomesticCorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ForeignCorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: