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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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	$ (11,711)	$ (6,950)	$ (31,617)	$ (23,043)
Commercial Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(3,500)	(5,200)	(17,800)	(12,100)
Commercial Specialty | General Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(2,500)	(4,400)	(20,500)	(9,600)
Commercial Specialty | General Liability | Accident Years 2005 through 2009 and 2012 | Construction Defect
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(1,900)
Commercial Specialty | General Liability | Accident Year 1995, 2005, 2007 through 2012 and 2015 and 2016 through 2018 | Other General Liability
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 2004 Through 2012 and 2016 | Construction Defect
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(500)		(1,000)
Commercial Specialty | General Liability | Accident Years 1999 through 2014 and 2016 and 2017 | Other General Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(3,900)
Commercial Specialty | General Liability | Accident Years 2005 through 2009, 2012, 2015 through 2017 | Construction Defect
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(6,600)
Commercial Specialty | General Liability | Accident Years 2005 through 2016 and 2019 | Other General Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(13,900)
Commercial Specialty | General Liability | Accident Years 1999 Through 2018 | Other General Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(8,600)
Commercial Specialty | Property Lines | Accident Years 2017 through 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	100		5,800
Commercial Specialty | Property Lines | Accident Years 2010, 2012 Through 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(900)
Commercial Specialty | Professional Liability | Accident Years 2006 through 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(1,100)
Commercial Specialty | Professional Liability | Accident Years 2007 through 2010 and 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(1,900)
Commercial Specialty | Professional Liability | Accident Years 2009 and 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(1,100)
Commercial Specialty | Commercial Auto Liability | Accident Years 2012,2013 and 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(600)
Commercial Specialty | Commercial Auto Liability | Accident Years 2010 and 2012 through 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(1,000)
Commercial Specialty | Commercial Auto Liability | Accident Years 2000 Through 2002, 2010, 2012, and 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(1,400)
Commercial Specialty | Workers Compensation | Accident Years 2012 and prior to 2005
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(200)
Commercial Specialty | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				1,000
Reinsurance Operations
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(4,900)	700	(5,100)	3,500
Reinsurance Operations | Property Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				3,800
Reinsurance Operations | Property Lines | Accident years 2009 through 2012 and 2014 through 2018 and 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(3,200)
Reinsurance Operations | Property Lines | Accident Years 2018 | Typhoon Jebi
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				7,600
Reinsurance Operations | Property Lines | Accident Years 2011, 2012, and 2014 through 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		700
Reinsurance Operations | Property Lines | Accident years 2009 through 2010 and 2012 through 2017 and 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(3,400)
Reinsurance Operations | Property Lines | Accident Years 2010 Through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(4,100)
Reinsurance Operations | Professional Liability | Accident years 2014 and 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(1,700)		(1,700)
Reinsurance Operations | Professional Liability | Accident Years 2007, 2008 and 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(300)
Specialty Property
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(2,000)	(1,300)	(6,600)	(10,500)
Specialty Property | General Liability | Accident Years 2017 and 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(200)
Specialty Property | General Liability | Accident Years 2010, 2014 through 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(400)
Specialty Property | General Liability | Accident Years 2015 through 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(2,000)
Specialty Property | General Liability | Accident Years 2010 and 2015 through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(400)
Specialty Property | Property Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(900)	(4,600)	(10,100)
Specialty Property | Property Lines | Accident Years 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(200)
Specialty Property | Property Lines | Accident Years 2017 | Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(1,800)		(1,800)
Specialty Property | Property Lines | Accident Years 2018 | Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(600)		(8,900)
Specialty Property | Property Lines | Accident Years 2015 through 2018 and 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(2,800)
Specialty Property | Property Lines | Accident Years 2016 and 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(1,200)
Farm, Ranch & Stable
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(1,300)	(1,200)	(2,100)	(4,000)
Farm, Ranch & Stable | General Liability | Accident years 2013 and 2015 through 2016 and 2018 through 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(100)
Farm, Ranch & Stable | Property Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(2,200)
Farm, Ranch & Stable | Property Lines | Accident Years 2017 through 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	$ (1,300)
Farm, Ranch & Stable | Property Lines | Accident Years 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		$ (1,100)		(2,000)
Farm, Ranch & Stable | Property Lines | Accident Years 2018 | Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(1,100)
Farm, Ranch & Stable | Property Lines | Accident Years 2018 | Expected Claims Frequency And Severity
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(900)
Farm, Ranch & Stable | Property Lines | Accident Year 2016 Through 2019 | Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			$ (2,000)
Farm, Ranch & Stable | Liability Lines | Accident Years 2013 through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				$ (1,700)

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No definition available.

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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

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