Noncash Investing and Financing Activities (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Issuance of common stock — net share settlement of employee options	$ 45	$ 43

Stock issued in noncash financing activity where options were exercised via the net share settlement method. Amount represents the strike price of options exercised.

No definition available.

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