Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

No definition available.

+ Details

Name:	us-gaap_RepaymentsOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_DebtInstrumentAxis=trv_FixedToFloatingRateJuniorSubordinatedDebentures6.25PercentDueMarch2067Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=trv_SeniorNotes5.75PercentDueDecember2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=trv_SeniorNotes6.25PercentDueJune2016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=trv_SeniorNotes5.50PercentDueDecember2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt (details) - Repayment - USD ($) $ in Millions					12 Months Ended
Debt (details) - Repayment - USD ($) $ in Millions	Dec. 15, 2017	Jun. 02, 2017	Jun. 20, 2016	Dec. 01, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument
Debt, principal amount repaid					$ 657	$ 400	$ 400
6.25% Fixed-to-floating rate junior subordinated debentures due March 15, 2067 [Member]
Debt Instrument
Debt, principal amount repaid		$ 107
Interest rate (percent)		6.25%				6.25%
Percentage of principal amount at which redemption price was set		100.00%
5.75% Senior notes due December 15, 2017 [Member]
Debt Instrument
Debt, principal amount repaid	$ 450
Interest rate (percent)	5.75%					5.75%
6.25% Senior notes due June 20, 2016 [Member]
Debt Instrument
Debt, principal amount repaid			$ 400
Interest rate (percent)			6.25%
5.50% Senior notes due December 1, 2015 [Member]
Debt Instrument
Debt, principal amount repaid				$ 400
Interest rate (percent)				5.50%