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Fair value measurements Fair Value Measurements (narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |
Fair Value, Level 1 to Level 2 transfers, Amount | $ 0 | $ 0 | $ 0 | $ 0
Fair Value, Level 2 to Level 1 Transfers, Amount | 0 | $ 0 | $ 0 | $ 0
Realized Gain Losses As A Result of Annuitization Changes | $ 94 |  |  |
Minimum [Member] |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |
Notice Period For Redemption For Alternative Investments Investment Funds |  |  | 5 days |
Maximum [Member] |  |  |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |  |  |
Notice Period For Redemption For Alternative Investments Investment Funds |  |  | 120 days |

X
- Definition Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. + References No definition available. + Details
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Namespace Prefix: | cb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1. + References No definition available. + Details
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Namespace Prefix: | cb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Notice period in days for redemption of investment funds alternative investments (employ various investment strategies such as long/short equity and arbitrage/distressed.) + References No definition available. + Details
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X
- Definition Realized Gain Losses As A Result of Annuitization Changes + References No definition available. + Details
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X
- 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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- Details
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Period Type: |

X
- Details
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Period Type: |