Investments (Invested Assets on Deposit, Held In Trust and Pledged as Collateral) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Invested assets on deposit (regulatory deposits)	$ 1,804	$ 1,879
Invested assets held in trust (collateral financing arrangement and reinsurance agreements)	3,100	2,490
Invested assets pledged as collateral	24,808	24,174
Total invested assets on deposit, held in trust and pledged as collateral	$ 29,712	$ 28,543

The sum of the fair values, as of the date of each statement of financial position presented, of securities that are restricted as collateral for the payment of related debt obligations, primarily secured borrowings and repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

No definition available.

+ Details

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Namespace Prefix:	met_
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Balance Type:	debit
Period Type:	instant

- Definition

Invested assets pledged as collateral.

+ References

No definition available.

+ Details

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

- Definition

The total amount of cash and securities held by third party trustees pursuant to terms of debt instruments or other agreements as of the date of each statement of financial position presented, which can be used by the trustee only to pay the noncurrent portion of specified obligations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(b))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The carrying amount of the asset transferred to a third party to serve as a deposit, which typically serves as security against failure by the transferor to perform under terms of an 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 210.5-02.8,17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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