Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Percent of fixed maturities for which a pricing service estimates fair value	98.00%	98.00%
Fair value of the fixed maturities for which the Company used an internal pricing matrix	$ 62	$ 127
Fair value of the fixed maturities for which the Company received a broker quote	$ 139	$ 77

The fair value of the fixed maturities for which the Company used an internal pricing matrix in determining fair value.

No definition available.

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