Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Percent of fixed maturities for which a pricing service estimates fair value	99.00%	99.00%
Estimated fair value of put/call option	$ 4	$ 5

The estimated fair value of a put/call option that was entered into in connection with a business acquisition that allows the Company to acquire the remaining shares of the acquired company at a future date.

No definition available.

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