Liability for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	$ (9,417)	$ (8,512)	$ (24,934)	$ (17,171)
Commercial Lines Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(6,500)	(7,300)	(18,800)	(12,900)
Commercial Lines Segment | Umbrella Lines | Accident Years 2003 through 2005
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 2011 through 2014
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 2011 through 2015
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 | Property Lines | Accident years 2012 through 2015
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 2012 through 2015 | Non-catastrophe [Member]
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 2012 through 2015 | Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			300
Commercial Lines Segment | General Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(9,000)		(21,100)
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	(3,200)		(4,800)
Commercial Lines Segment | General Liability | Accident years 2004 through 2012 | Other General Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(6,000)		(16,300)
Commercial Lines Segment | General Liability | Accident Years 1999 through 2013 | Construction Defect
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(3,800)
Commercial Lines Segment | General Liability | Accident Years Prior to 2013 | Construction Defect
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(5,700)
Commercial Lines Segment | Professional | Accident Years 2006 through 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(3,500)		(6,400)
Reinsurance Operations
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(2,900)	(1,200)	(6,100)	(4,300)
Reinsurance Operations | Property Lines | Accident Years 2009 to 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		$ (1,200)
Reinsurance Operations | Property Lines | 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	$ (2,900)
Reinsurance Operations | Property Lines | Accident years 2013 through 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			$ (6,100)
Reinsurance Operations | Property Lines | Accident Years 2009 to 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				$ (4,300)