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Unearned revenue liability (URL Rollforward) (Details) - USD ($) $ in Millions | 9 Months Ended
--- | ---
Sep. 30, 2025 | Sep. 30, 2024
Insurance [Abstract] |  |
Balance – beginning of period | $ 711 | $ 673
Deferred revenue | 101 | 106
Amortization | (58) | (54)
Other (including foreign exchange) | 6 | (13)
Balance – end of period | $ 760 | $ 712

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- Definition Unearned Revenue Liability + References No definition available. + Details
Name: | cb_UnearnedRevenueLiability
Namespace Prefix: | cb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Unearned Revenue Liability, Amortization + References No definition available. + Details
Name: | cb_UnearnedRevenueLiabilityAmortization
Namespace Prefix: | cb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Unearned Revenue Liability, Deferred Revenue + References No definition available. + Details
Name: | cb_UnearnedRevenueLiabilityDeferredRevenue
Namespace Prefix: | cb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Unearned Revenue Liability, Other Increase (Decrease) + References No definition available. + Details
Name: | cb_UnearnedRevenueLiabilityOtherIncreaseDecrease
Namespace Prefix: | cb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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