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Investments (Schedule Of Default Assumptions By Moody's Rating Categories) (Details) | 12 Months Ended
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Dec. 31, 2017
Investment Grade | Aaa - Bbb | Minimum |
Financing Receivable, Recorded Investment [Line Items] |
1-in-100 Year Default Rate | 0.00%
Historical Mean Default Rate | 0.00%
Investment Grade | Aaa - Bbb | Maximum |
Financing Receivable, Recorded Investment [Line Items] |
1-in-100 Year Default Rate | 1.30%
Historical Mean Default Rate | 0.30%
Below Investment Grade | Ba |
Financing Receivable, Recorded Investment [Line Items] |
1-in-100 Year Default Rate | 4.80%
Historical Mean Default Rate | 1.00%
Below Investment Grade | B |
Financing Receivable, Recorded Investment [Line Items] |
1-in-100 Year Default Rate | 12.10%
Historical Mean Default Rate | 3.20%
Below Investment Grade | Caa - C |
Financing Receivable, Recorded Investment [Line Items] |
1-in-100 Year Default Rate | 36.80%
Historical Mean Default Rate | 10.50%

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- Definition Moody's rating category one in hundred year default rate. + References No definition available. + Details
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Namespace Prefix: | cb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Moody's rating category historical mean default rate. + References No definition available. + Details
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Namespace Prefix: | cb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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

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- Details
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Data Type: | na
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- Details
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