Losses and Loss Expenses Payable - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	$ 46.6	$ (27.0)	$ (10.0)
Unallocated loss adjustment expense [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	3.2	(3.7)	6.2
Personal Insurance Segments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	1.6	(9.0)	10.5
Personal Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	4.4	(7.9)	11.1
Homeowners [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	1.5
Other Personal [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	1.3
Commercial insurance segment
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	44.0	5.1	0.7
Commercial Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		(3.6)	(9.8)
Workers' compensation [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		4.2	5.1
Farm & Ranch [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		1.7
Other Commercial [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		1.8
Specialty Insurance Segment [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(4.2)	(20.8)	7.1
Programs [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		(14.3)	9.6
Excess & Surplus Property [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(3.0)		5.2
Excess & Surplus Casualty [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		(4.6)	2.7
Middle Market Commercial [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years			4.3
Catastrophe [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	$ 2.0	$ 1.4	$ 0.7