Losses and Loss Expenses Payable - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	$ 10,000,000	$ 45,100,000	$ (21,200,000)
Unallocated loss adjustment expense [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(6,200,000)	(12,500,000)	(8,000,000)
Catastrophe [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(700,000)	(5,200,000)	(5,400,000)
Personal And Business Insurance Segments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(9,800,000)	(28,700,000)	(30,500,000)
Personal Insurance Segments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(9,400,000)	(6,400,000)
Personal Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(11,000,000)
Business insurance segment
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(400,000)	(22,300,000)
Commercial Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(10,500,000)	(5,300,000)	(8,000,000)
Workers' compensation [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(5,100,000)	(5,600,000)	(12,300,000)
Product Liability [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(3,800,000)	(11,900,000)	(8,300,000)
Specialty Insurance Segment [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(7,100,000)	(91,500,000)	(22,700,000)
Commercial Multi-Peril [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		(2,100,000)
Programs [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(9,700,000)
Excess & Surplus Property [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(5,300,000)	(3,900,000)
Excess & Surplus Casualty [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	$ (2,700,000)	(3,100,000)
RED [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		$ (96,700,000)	(21,300,000)
Homeowners [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years			(2,900,000)
Specialty Insurance Segment [Member] | Workers' compensation [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years			$ (0.3333)