Investments (Purchased Financial Assets with Credit Deterioration) (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019
Purchased Financial Assets with Credit Deterioration [Line Items]
ACL at Acquisition	$ (16)	$ 0
Residential Mortgage Loans
Purchased Financial Assets with Credit Deterioration [Line Items]
Contractual Principal	512
ACL at Acquisition	(16)	$ 0
Non-Credit (Discount) Premium	(18)
Purchase Price	$ 478

[Line Items] for Purchased Financial Assets with Credit Deterioration [Table]

No definition available.

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

- Definition

Amount of allowance for credit loss at acquisition date of financing receivable purchased with credit deterioration.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 19
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919265-210447

+ Details

Name:	us-gaap_FinancingReceivablePurchasedWithCreditDeteriorationAllowanceForCreditLossAtAcquisitionDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount at par value of financing receivable purchased with credit deterioration.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 19
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919265-210447

+ Details

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

- Definition

Amount at purchase price of financing receivable purchased with credit deterioration.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 19
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919265-210447

+ Details

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

- Definition

Amount of discount (premium) attributable to other factors on financing receivable purchased with credit deterioration.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 19
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919265-210447

+ Details

Name:	us-gaap_FinancingReceivablePurchasedWithCreditDeteriorationDiscountPremium
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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