Investments (details) - Trust Deposits - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Subsidiary [Member]
Investment [Line Items]
Fair value of owned securities pledged into a Lloyd's trust account to support capital requirements for the Company's operations at Lloyd's	$ 37	$ 97
Travelers [Member]
Investment [Line Items]
Fair value of owned securities pledged into a Lloyd's trust account to support capital requirements for the Company's operations at Lloyd's	$ 33

The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to a trust deposit to support capital requirements for the Company's operations at Lloyd's, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

No definition available.

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