Investments - Restricted Assets (Details) $ in Thousands	Dec. 31, 2019 USD ($) syndicate	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Investments, Debt and Equity Securities [Abstract]
Restricted cash and cash equivalents	$ 352,692	$ 380,488	$ 257,686
Line of Credit Facility [Line Items]
Collateral in trust for third party agreements	4,104,093	4,336,752
Assets on deposit with regulatory authorities	445,626	579,048
Funds at Lloyd's	639,316	354,589
Restricted assets, total	$ 5,322,273	5,398,230
Number of syndicates \| syndicate	3
Asset Pledged as Collateral
Line of Credit Facility [Line Items]
Collateral for secured letter of credit facilities	$ 133,238	$ 127,841

Funds in Syndicate

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Restricted Cash and Investments, Number of Syndicates

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The total amount of cash and securities held by third party trustees pursuant to terms of debt instruments or other agreements as of the date of each statement of financial position presented, which can be used by the trustee only to pay the noncurrent portion of specified obligations.

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Amount of restricted investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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The carrying amount of the asset transferred to a third party to serve as a deposit, which typically serves as security against failure by the transferor to perform under terms of an agreement.

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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.

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Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Cash and investments whose use in whole or in part is restricted for the long-term, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

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