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Liability for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	$ (51)	$ (2,992)	$ 0	$ (9,626)
Commercial Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	14,500	(1,200)	21,000	(1,600)
Commercial Specialty | Property Lines | Accident Years 2020 and 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(1,800)
Commercial Specialty | Property Lines | Accident Years 2019 through 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(3,300)
Commercial Specialty | Property Lines | Accident Years 2015 and 2018 through 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	2,200		1,400
Commercial Specialty | General Liability | Accident Years Prior to 2005 through 2018 2019 through 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	12,400
Commercial Specialty | General Liability | Accident Years Prior to 2005 through 2018 | Construction Defect Segments
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(6,500)
Commercial Specialty | General Liability | Accident Years Prior to 2005, 2005, 2013, 2014, 2016 through 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		500
Commercial Specialty | General Liability | Accident Years 2013 and 2019 through 2022 prior to 2013 and 2014 through 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			19,700
Commercial Specialty | General Liability | Accident Years prior to 2013 and 2014 through 2018 | Construction Defect Segments
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(8,600)
Commercial Specialty | General Liability | Accident Years Prior to 2005, 2006, 2007, 2011 through 2015, and 2017 through 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				1,700
Commercial Specialty | Professional | Accident Years prior to 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(100)
Commercial Specialty | Professional | Accident Years prior to 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(100)
Reinsurance Operations
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(2,700)	(1,200)	(1,700)	(2,500)
Reinsurance Operations | General Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(100)
Reinsurance Operations | Professional | Accident Year 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(1,200)		(2,400)
Reinsurance Operations | Professional | Accident Years 2020 2017 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(2,600)
Reinsurance Operations | Professional | Accident Years 2016, 2022, 2017, and 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(1,500)
Exited Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(11,900)	(600)	(19,300)	(5,500)
Exited Lines | Property Lines | Accident Years 2013 through 2020 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(10,000)
Exited Lines | Property Lines | Accident Years 2016, 2020 and 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		2,200
Exited Lines | Property Lines | Accident Years 2013 through 2020, 2021 and 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(15,900)
Exited Lines | Property Lines | Accident Years 2016, 2017, 2020 and 2021 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				2,000
Exited Lines | General Liability | Accident Years 2017, 2018, 2019 and 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(600)
Exited Lines | General Liability | Accident Years 2014, 2016 through 2019, 2021 and 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	$ (1,800)
Exited Lines | General Liability | Accident Years 2012 through 2019, 2021 and 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			$ (3,400)
Exited Lines | Reinsurance | Accident Years 2017 through 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				$ (7,000)
Exited Lines | Reinsurance | Accident Years 2016 through 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		$ (2,700)

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Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480081/944-40-50-3

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