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v3.19.1

Fair value measurements Fair Value Measurements (narrative) (Details) | 3 Months Ended
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Mar. 31, 2019
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 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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Namespace Prefix: | cb_
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Period Type: | duration

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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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |