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Allowance for Expected Credit Losses (Details) - Changes in the Allowance for Expected Credit Losses on Contractholder Receivables (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Credit Loss [Abstract] |  |  |  |
Contractholder receivables (net of the allowance for expected credit losses), balance, beginning of period | $ 3,095 | $ 3,274 | $ 3,171 | $ 3,249
Contractholder receivables (net of the allowance for expected credit losses), balance, end of period | 3,098 | 3,292 | 3,098 | 3,292
Contractholder receivables, allowance for expected credit losses [Roll Forward] |  |  |  |
Contractholder receivables, allowance for expected credit losses, beginning balance | 17 | 18 | 18 | 20
Current period change for expected credit losses | 0 | 0 | (1) | (2)
Write-offs of uncollectible contractholder receivables | 0 | 0 | 0 | 0
Contractholder receivables, allowance for expected credit losses, ending balance | $ 17 | $ 18 | $ 17 | $ 18

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- Definition Under certain workers' compensation insurance contracts with deductible features, the Entity has an obligation to pay the full amount claimed. The Entity is subsequently reimbursed by the policyholder for the policy deductible. The amount to be reimbursed is included on a gross basis in the consolidated balance sheet in contractholder receivables and contractholder payables, respectively. + References No definition available. + Details
Name: | trv_ContractholderReceivables
Namespace Prefix: | trv_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Amount of allowance for credit loss on contractholder receivables. + References No definition available. + Details
Name: | trv_ContractholderReceivablesAllowanceForCreditLoss
Namespace Prefix: | trv_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
Name: | trv_ContractholderReceivablesAllowanceForCreditLossRollForward
Namespace Prefix: | trv_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of expense (reversal of expense) for expected credit loss on contractholder receivables. + References No definition available. + Details
Name: | trv_ContractholderReceivablesCreditLossExpenseReversal
Namespace Prefix: | trv_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of the write-off of uncollectible contractholder receivables. + References No definition available. + Details
Name: | trv_ContractholderReceivablesLossOnUncollectibleAccountsInPeriod
Namespace Prefix: | trv_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_CreditLossAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration