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Cash transferred

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Retrocede amount

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Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

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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 liabilities, classified as other, assumed in acquiring a business or in consideration for an asset received in a noncash or part noncash acquisition.

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Aggregate par or stated value of issued nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer). This item includes treasury stock repurchased by the entity. Note: elements for number of nonredeemable preferred shares, par value and other disclosure concepts are in another section within stockholders' equity.

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Amount before valuation allowance recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

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The amount of an asset, typically cash, provided to a counterparty to provide certain assurance of performance by the entity pursuant to the terms of a written or oral agreement, such as a lease.

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Percentage of the Variable Interest Entity's (VIE) voting interest owned by (or beneficial interest in) the reporting entity (directly or indirectly).

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Collateralized Reinsurance And Trust Agreements (Narrative) (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
	Apr. 01, 2018	Sep. 30, 2021	Dec. 31, 2020
Effects of Reinsurance [Line Items]
Amount on deposit in trust accounts		$ 1,526.1
Preferred shares
Reinsurance recoverable		217.5	254.9
Deferred gain liabliity		$ 34.4	38.8
Mt Logan Re [Member]
Effects of Reinsurance [Line Items]
Percentage of voting common shares owned by Group		100.00%
Assumed net risk exposures		$ 847.3	806.6
Net risk exposure investments		$ 61.6	$ 67.6
Retrocede amount	$ 269.2
Cash transferred	252.0
Mt Logan Re [Member] \| Minimum [Member]
Effects of Reinsurance [Line Items]
Accident year		2002
Mt Logan Re [Member] \| Maximum [Member]
Effects of Reinsurance [Line Items]
Accident year		2015
Liability retroceded	$ 319.0