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Amount of written premiums ceded to other entities.

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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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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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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) $ in Thousands								1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 12, 2019 USD ($) Item	Apr. 30, 2018 USD ($) Item	Apr. 01, 2018 USD ($)	Apr. 13, 2017 USD ($) Item	Dec. 01, 2015 USD ($) Item	Nov. 18, 2014 USD ($)	Apr. 24, 2014 USD ($) Item	Nov. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Effects of Reinsurance [Line Items]
Amount on deposit in trust accounts									$ 1,158,783		$ 1,158,783
Reinsurance coverage amount									86,712	$ 97,391	$ 245,422	$ 237,841
Catastrophe Reinsurance Agreement 3 [Member]
Effects of Reinsurance [Line Items]
Reinsurance coverage amount				$ 325,000
Catastrophe Reinsurance Agreement 4 [Member]
Effects of Reinsurance [Line Items]
Reinsurance coverage amount				50,000
Catastrophe Reinsurance Agreement 5 [Member]
Effects of Reinsurance [Line Items]
Reinsurance coverage amount				75,000
Catastrophe Reinsurance Agreement 6 [Member]
Effects of Reinsurance [Line Items]
Reinsurance coverage amount				$ 175,000
Catastrophe Reinsurance Series 2015-1 Notes [Member]
Effects of Reinsurance [Line Items]
Catastrophe reinsurance bonds funded amount					$ 625,000
Kilimanjaro Re Limited [Member]
Effects of Reinsurance [Line Items]
Number of collateralized reinsurance agreements \| Item				6	2		2
Reinsurance agreements expiration date							Apr. 30, 2018
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance [Member]
Effects of Reinsurance [Line Items]
Number of collateralized reinsurance agreements \| Item	4	4
Reinsurance coverage amount						$ 500,000
Reinsurance agreements expiration date						Nov. 30, 2019
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Agreement 1 Through 3 [Member]
Effects of Reinsurance [Line Items]
Number of collateralized reinsurance agreements \| Item				3
Reinsurance contract term				4 years
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Agreement 1 And 2 [Member]
Effects of Reinsurance [Line Items]
Number of collateralized reinsurance agreements \| Item	2	2
Reinsurance contract term	4 years	4 years
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Agreement 3 And 4 [Member]
Effects of Reinsurance [Line Items]
Number of collateralized reinsurance agreements \| Item	2	2
Reinsurance contract term	5 years	5 years
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Agreement 4 Through 6 [Member]
Effects of Reinsurance [Line Items]
Number of collateralized reinsurance agreements \| Item				3
Reinsurance contract term				5 years
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Agreement 1 [Member]
Effects of Reinsurance [Line Items]
Reinsurance coverage amount	$ 150,000	$ 62,500		$ 225,000	$ 300,000		$ 250,000
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Agreement 2 [Member]
Effects of Reinsurance [Line Items]
Reinsurance coverage amount	275,000	200,000		400,000	$ 325,000		200,000
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Agreement 3 [Member]
Effects of Reinsurance [Line Items]
Reinsurance coverage amount	150,000	62,500
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Agreement 4 [Member]
Effects of Reinsurance [Line Items]
Reinsurance coverage amount	275,000	200,000
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Series 2014-1 Notes [Member]
Effects of Reinsurance [Line Items]
Catastrophe reinsurance bonds funded amount							$ 450,000
Catastrophe Reinsurance Bonds Redeemed Amount		450,000
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Series 2014-2 Notes [Member]
Effects of Reinsurance [Line Items]
Catastrophe reinsurance bonds funded amount						$ 500,000
Catastrophe Reinsurance Bonds Redeemed Amount								$ 500,000
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Series 2017-1 Notes [Member]
Effects of Reinsurance [Line Items]
Catastrophe reinsurance bonds funded amount				950,000
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Series 2017-2 Notes [Member]
Effects of Reinsurance [Line Items]
Catastrophe reinsurance bonds funded amount				$ 300,000
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Series 2018-1 Notes [Member]
Effects of Reinsurance [Line Items]
Catastrophe reinsurance bonds funded amount		262,500
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Series 2018-2 Notes [Member]
Effects of Reinsurance [Line Items]
Catastrophe reinsurance bonds funded amount		$ 262,500
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Series 2019-1 Notes [Member]
Effects of Reinsurance [Line Items]
Catastrophe reinsurance bonds funded amount	425,000
Kilimanjaro Re Limited [Member] \| Catastrophe Reinsurance Series 2019-2 Notes [Member]
Effects of Reinsurance [Line Items]
Catastrophe reinsurance bonds funded amount	$ 425,000
Mt. Logan Re [Member]
Effects of Reinsurance [Line Items]
Percentage of voting common shares owned by Group											100.00%
Assumed net risk exposures												$ 806,564	$ 993,036
Net risk exposure investments									$ 66,175		$ 66,175		$ 46,390
Retrocede amount			$ 269,198
Cash transferred			252,000
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			$ 319,000