Liability for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	$ (57,252)	$ (34,698)	$ (16,433)
Unpaid losses and loss adjustment expense reserves	651,042	680,047	675,472	$ 779,466
Unpaid losses and loss adjustment expense reserves, net	520,603	571,917	552,271	586,975
Unpaid losses and loss adjustment expenses reserves	$ 29,890	$ 30,529	$ 31,185	$ 23,038
Survival ratio on a gross basis for open A&E claims, periods	13 years 9 months 18 days	15 years	10 years 9 months 18 days
Survival ratio on net basis for open A&E claims, periods	19 years 3 months 18 days	16 years 9 months 18 days	8 years 4 months 24 days
Construction Defect
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expense reserves	$ 54,500	$ 70,400
Unpaid losses and loss adjustment expense reserves, net	48,600	62,200
IBNR Reserves | Asbestos
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses reserves	26,700	26,000	$ 26,400
Case Reserves | Asbestos
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss adjustment expenses reserves	3,200	4,500	4,800
Commercial Lines Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(43,800)	(25,600)	(12,500)
Commercial Lines Segment | Property Lines | Accident Years 2009, 2012, 2015, 2008, 2011, and 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	800
Commercial Lines Segment | Property Lines | Accident Years 2009, 2012, 2015, 2008, 2011, and 2013 | Non-catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	500
Commercial Lines Segment | Property Lines | Accident Years 2009, 2012, 2015, 2008, 2011, and 2013 | Catastrophe segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	300
Commercial Lines Segment | Property Lines | Accident Years 2007, 2012 and 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			2,100
Commercial Lines Segment | Marine | Accident Years 2010 through 2012 | Casualty Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(1,400)
Commercial Lines Segment | General Liability | Casualty Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(43,800)	(20,400)
Commercial Lines Segment | General Liability | Accident years 2005 through 2015 | Construction Defect
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(9,400)
Commercial Lines Segment | General Liability | Accident years 2004 through 2014 | Other General Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(34,400)	(14,500)
Commercial Lines Segment | General Liability | Accident years 2008 through 2014 | Construction Defect
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(5,900)
Commercial Lines Segment | General Liability | Accident Years 2001, 2007 through 2010, and 2013 | Casualty Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(3,100)
Commercial Lines Segment | Professional | Accident years 2006 through 2011 | Casualty Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(6,200)
Commercial Lines Segment | Professional | Accident Years 2007 through 2011 | Casualty Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(19,400)
Commercial Lines Segment | Asbestos and Environmental | Accident Years Prior to 1990 | Casualty Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			7,100
Commercial Lines Segment | Umbrella Lines | Accident Years 2002 through 2007 | Casualty Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(2,700)
Commercial Lines Segment | Commercial Auto Segment | Accident years 2011 through 2013 | Casualty Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			3,600
Reinsurance Operations
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(13,500)	(9,100)	$ (3,900)
Reinsurance Operations | Property Lines | Accident years 2010 through 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	$ (13,500)
Reinsurance Operations | Property Lines | Accident years 2011 through 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(6,800)
Reinsurance Operations | Marine | Accident years 2010 and 2011
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(2,800)
Reinsurance Operations | Workers Compensation | Accident years 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		$ 1,000