Period over which an employee vests in the employer matching contribution.

No definition available.

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Namespace Prefix:	trv_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum amount the employer may match per employee contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	trv_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The percentage which the employee vests in employer matching contributions after the vesting period expires.

+ References

No definition available.

+ Details

Name:	trv_DefinedContributionPlanVestingPercentageAfterVestingPeriod
Namespace Prefix:	trv_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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

- Details

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Namespace Prefix:
Data Type:	na
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- Details

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Pension Plans, Retirement Benefits and Savings Plans (details) - Savings Plan - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Savings Plan
Expense related to all savings plans	$ 119,000,000	$ 114,000,000	$ 109,000,000
The Travelers 401(k) Savings Plan [Member]
Savings Plan
Maximum annual match	$ 6,500
Vesting percentage of employer match after vesting period	100.00%
Vesting period	3 years
The Travelers 401(k) Savings Plan [Member] \| Maxiumum [Member]
Savings Plan
Matching contribution as a percent of eligible pay	5.00%