Contingencies, Commitments and Guarantees (Details) - Guarantees $ in Millions	Jun. 30, 2018 USD ($)
Indemnifications related to the sale of businesses [Member]
Guarantor obligations
Maximum amount of contingent obligation	$ 358
Amount recognized on balance sheet for contingent obligation	2
Guarantees of certain investments and third-party loans [Member]
Guarantor obligations
Maximum amount of contingent obligation	45
Amount indemnified by a third party	23
Guarantees of certain insurance policy obligations [Member]
Guarantor obligations
Maximum amount of contingent obligation	480
Amount indemnified by a third party	$ 480

The amount of a guarantee that is indemnified by a third party.

No definition available.

Name:	trv_GuarantorObligationsAmountRecoverableFromThirdParty
Namespace Prefix:	trv_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248

+ Details

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

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248

+ Details

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

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_IndemnificationGuaranteeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=trv_PaymentGuaranteeCertainInvestmentsAndThirdPartyLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=trv_PaymentGuaranteeCertainInsurancePolicyObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: