Insurance Claim Reserves (Details) - Subrogation Amounts $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($)
Insurance Loss Reserves [Abstract]
Subrogation benefit recognized in earnings	$ 403
Subrogation payments received	$ 380

Payments received 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_SubrogationRecoveryPaymentsReceived
Namespace Prefix:	trv_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration