Liability for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	$ (8,921)	$ (5,164)	$ (15,517)	$ (8,659)
Commercial Lines Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(6,700)	(3,600)	(12,300)	(5,600)
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 | Umbrella Lines | Accident years 2004 and 2011
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	700		700
Commercial Lines Segment | Property Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(1,200)
Commercial Lines Segment | Property Lines | Accident Years 2009 through 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 2008 through 2013 and 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(1,500)
Commercial Lines Segment | Property Lines | Accident years 2012 and 2015
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 2012, 2014 and 2015
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	(7,900)		(12,100)
Commercial Lines Segment | General Liability | Accident Years 2011 and 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(1,100)		(1,800)
Commercial Lines Segment | General Liability | Accident years 2011 through 2015 | Construction Defect
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(2,000)		(1,600)
Commercial Lines Segment | General Liability | Accident years 2007 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	(5,900)		(10,500)
Commercial Lines Segment | Professional | Accident years 2006 through 2011
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(2,900)		(2,900)
Reinsurance Operations
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(2,200)	(1,600)	(3,200)	(3,100)
Reinsurance Operations | Property Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(2,200)		(3,200)
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		$ (1,600)		$ (3,100)
Reinsurance Operations | Property Lines | Accident years 2013 and 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(3,300)		(4,000)
Reinsurance Operations | Property Lines | Accident year 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	$ 1,100		$ 800