Liability for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	$ (5,164)	$ (4,028)	$ (8,659)	$ (6,122)
Commercial Lines Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(3,600)	(3,000)	(5,600)	(5,000)
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		800
Commercial Lines Segment | General Liability | Ongoing General Liability Book
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(6,000)
Commercial Lines Segment | General Liability | Older Environmentally Exposed Book
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		6,800
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 | Other Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(700)
Commercial Lines Segment | Property Lines | Accident Years Two Thousand Nine Through Two Thousand Thirteen
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(800)
Commercial Lines Segment | Professional | Accident Years 2007 through 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(10,000)		(11,800)
Commercial Lines Segment | Professional | Accident Years Two Thousand Six Through Two Thousand Eleven
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(2,900)		(2,900)
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 | Asbestos | Accident Years Prior to 1990
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		6,900		6,900
Reinsurance Operations
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve	(1,600)	(1,000)	(3,100)	(1,100)
Reinsurance Operations | 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 Year 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(1,300)
Reinsurance Operations | Property Lines | 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				(1,300)
Reinsurance Operations | Commercial Auto Liability | Accident Years 2009Through 2011
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		$ 300		$ 200