Contingent Liabilities - Narrative (Details)	Sep. 27, 2021 USD ($)	Jul. 01, 2021 USD ($)	Jun. 30, 2022 USD ($) credit_facility	Dec. 31, 2021 USD ($)
Contingent Liabilities [Line Items]
Reinsurance recoverable on unpaid losses, net			$ 1,570,885,000	$ 1,348,628,000
Reinsurance recoverable on paid losses			106,509,000	$ 82,235,000
Excess and Surplus Lines | Commercial Auto Business | Raiser | Indemnity Trust
Contingent Liabilities [Line Items]
Cash equivalent collateral returned to trust	$ 691,300,000
Cash equivalent collateral trust arrangement			374,300,000
Required collateral, percentage	102.00%
Excess and Surplus Lines | Commercial Auto Business | Raiser | Indemnity Trust And Loss Fund Trust
Contingent Liabilities [Line Items]
Cash equivalent collateral trust arrangement			443,400,000
Excess and Surplus Lines | Commercial Auto Business | Raiser | Loss Fund Trust
Contingent Liabilities [Line Items]
Cash equivalent collateral trust arrangement			69,200,000
Excess and Surplus Lines | Commercial Auto Business | Aleka | Loss Fund Trust
Contingent Liabilities [Line Items]
Cash equivalent collateral trust arrangement			28,200,000
Excess and Surplus Lines | Commercial Auto Business | Rasier and Aleka | Loss Fund Trust
Contingent Liabilities [Line Items]
Cash equivalent collateral trust arrangement			102,100,000
Loss Portfolio Transfer | Excess and Surplus Lines | Commercial Auto Business
Contingent Liabilities [Line Items]
Reinsurance premium		$ 345,100,000
Loss Portfolio Transfer | Excess and Surplus Lines | Commercial Auto Business | Aleka
Contingent Liabilities [Line Items]
Required collateral, percentage	102.00%
Reinsurance recoverables			188,900,000
Reinsurance recoverable on unpaid losses, net			170,300,000
Reinsurance recoverable on paid losses			18,600,000
Loss Portfolio Transfer | Excess and Surplus Lines | Commercial Auto Business | Aleka | L PT Trust
Contingent Liabilities [Line Items]
Cash equivalent collateral trust arrangement			165,400,000
Loss Portfolio Transfer | Excess and Surplus Lines | Commercial Auto Business | Aleka | LPT Trust And Loss Fund Trust
Contingent Liabilities [Line Items]
Cash equivalent collateral trust arrangement			$ 193,600,000
JRG Reinsurance Company, Ltd.
Contingent Liabilities [Line Items]
Number of credit facilities | credit_facility			3
JRG Reinsurance Company, Ltd. | Third-party Reinsureds
Contingent Liabilities [Line Items]
Assets held in trust			$ 417,400,000
JRG Reinsurance Company, Ltd. | Letter Of Credit 30 Million
Contingent Liabilities [Line Items]
Letters of credit facility, amount			30,000,000
Amount of letters of credit issued			4,600,000
Assets deposited for securing letters of credit			9,100,000
JRG Reinsurance Company, Ltd. | Letter of credit 102.5 Million
Contingent Liabilities [Line Items]
Letters of credit facility, amount			102,500,000
Amount of letters of credit issued			38,200,000
Assets deposited for securing letters of credit			46,600,000
JRG Reinsurance Company, Ltd. | Letter of credit 100 Million
Contingent Liabilities [Line Items]
Amount of letters of credit issued			22,600,000
Assets deposited for securing letters of credit			30,000,000
Line of credit (up to)			$ 100,000,000