Contingencies, Commitments and Guarantees (Details) - Commitments - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Unfunded commitments to private equity limited partnerships and real estate partnerships	$ 1,600	$ 1,560

The amount of unfunded commitments to private equity limited partnerships and real estate partnerships in which the Company invests.

No definition available.

Name:	trv_UnfundedCommitmentsToPrivateEquityLimitedPartnershipsRealEstatePartnershipsAndOther
Namespace Prefix:	trv_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant