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Liability for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	$ 51	$ (3,571)	$ 51	$ (6,634)
Commercial Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	5,000	(2,300)	6,500	(400)
Commercial Specialty | Property Lines | Accident Years 2015 and 2018 through 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(3,400)
Commercial Specialty | Property Lines | Accident years 2015, 2020 and 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(1,000)
Commercial Specialty | Property Lines | Accident Years 2015, 2018, 2019 and 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(1,500)
Commercial Specialty | Property Lines | Accident Yeas 2015, 2019, 2020, 2021 and 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(800)
Commercial Specialty | General Liability | Accident Years Prior to 2006 and 2017 through 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	6,100
Commercial Specialty | General Liability | Accident Years Prior to 2005 and 2005 through 2007 and 2010 through 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				1,200
Commercial Specialty | General Liability | Accident Years 2005 2010 through 2015, 2016, 2017, 2018, 2019, 2020 and 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		1,200
Commercial Specialty | General Liability | Accident Years Prior to 2006, 2011, 2013, 2015, 2017 and 2019 through 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			7,300
Reinsurance Operations
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	1,000	(1,200)	1,000	(1,200)
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)		(100)
Reinsurance Operations | Professional | Accident Year 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	1,100		1,100
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)		(1,200)
Exited Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(5,900)	(100)	(7,400)	(5,000)
Exited Lines | Property Lines | Accident Years 2011, 2015 through 2018 and 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		2,000
Exited Lines | Property Lines | Accident Years 2014, 2017, 2020, 2021 and 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	1,100
Exited Lines | Property Lines | Accident Years 2011, 2015 through 2017, 2018, 2020 and 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(200)
Exited Lines | Property Lines | Accident Years 2014, 2016, 2017, 2020, 2021 and 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			1,300
Exited Lines | General Liability | Accident Years 2014 through 2017 and 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(1,400)
Exited Lines | General Liability | Accident years 2014 through 2017 and 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(900)
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 | Reinsurance | Accident Years 2012 and 2015 through 2021
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	$ (6,200)		$ (7,300)
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		$ (2,100)		$ (4,300)

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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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