Insurance Claim Reserves (Details) - Subrogation Recovery $ in Millions	3 Months Ended
Sep. 30, 2020 USD ($)
Insurance Loss Reserves [Abstract]
Subrogation benefit recognized in earnings	$ 403

Benefit in earnings related to insurer's ability, after paying a claim under a policy, to recover the amount of the loss from another person or entity who is legally liable for it.

No definition available.

Name:	trv_SubrogationRecoveryBenefit
Namespace Prefix:	trv_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration