Investments (details) - Net Unrealized Investment Gains $ in Millions	Dec. 31, 2017 USD ($)
Investments disclosure
Total net unrealized gains on investment securities (after-tax)	$ 1,110
Net unrealized gains on investment securities reported in accumulated other comprehensive income (after-tax)	954
Adjustment reported in retained earnings due to the enactment of the Tax Cuts and Jobs Act of 2017	$ 158

After-tax impact of the enactment of the Tax Cuts and Jobs Act of 2017 related to unrealized gains on investment securities that is included in Retained Earnings as a result of recognition in the Statement of Income.

No definition available.

Name:	trv_TaxCutsAndJobsActOf2017AdjustmentRelatedtoUnrealizedInvestmentGainsLossesIncludedInRetainedEarningsDueToRecognitionInIncome
Namespace Prefix:	trv_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant