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Earnings per share Earnings Per Share( Narrative) (Detail) | 3 Months Ended
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Sep. 30, 2017 shares
Earnings Per Share [Abstract] |
Incremental Common Shares Not Attributable To Share Based Payment Due to Loss | 3,820,673

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- Definition IncrementalCommonSharesNotAttributableToShareBasedPaymentArrangementsDueToLoss + References No definition available. + Details
Name: | cb_IncrementalCommonSharesNotAttributableToShareBasedPaymentArrangementsDueToLoss
Namespace Prefix: | cb_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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