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Investment Information - Restricted assets (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Restricted Assets [Line Items] |  |
Restricted assets | $ 8,587,555 | $ 7,721,618
Collateral or guarantees - affiliated transactions |  |
Restricted Assets [Line Items] |  |
Restricted assets | 4,829,252 | 4,526,761
Collateral or guarantees - third party agreements |  |
Restricted Assets [Line Items] |  |
Restricted assets | 2,636,097 | 2,278,248
Deposits with US regulatory authorities |  |
Restricted Assets [Line Items] |  |
Restricted assets | 942,480 | 797,371
Deposits with non-US regulatory authorities |  |
Restricted Assets [Line Items] |  |
Restricted assets | 179,726 | 119,238
Watford Holdings Ltd |  |
Restricted Assets [Line Items] |  |
Assets pledged as collateral | $ 1,190,000 | $ 1,000,000

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- Definition Assets, consisting primarily of fixed maturities, that the Company is required to maintain with various regulatory authorities to support its insurance and reinsurance operations. The Company's insurance and reinsurance subsidiaries maintain assets in trust accounts as collateral for insurance and reinsurance transactions with affiliated companies and also have investments in segregated portfolios primarily to provide collateral or guarantees for letters of credit to third parties. + References No definition available. + Details
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Namespace Prefix: | acgl_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition -- None. No documentation exists for this element. -- + References No definition available. + Details
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Namespace Prefix: | acgl_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition Total assets that cannot be used for operating purposes because of contract or regulatory requirements that are in effect for a period that extends beyond one year. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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