Losses and Loss Expenses Payable - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	$ 80.2	$ 46.6	$ (27.0)
Personal and Commercial Insurance Segments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(79.7)
Small Commercial Package [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	9.6
Unallocated loss adjustment expense [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(7.1)	(3.2)	3.7
Personal Insurance Segments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		(1.6)	9.0
Personal Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(24.4)	(4.4)	7.9
Homeowners [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	7.3	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
Commercial Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	8.4		(3.6)
Workers' compensation [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	13.3		(4.2)
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	3.1	4.2	20.8
Programs [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(1.2)		(14.3)
Excess & Surplus Property [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		3.0
Excess & Surplus Casualty [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(2.7)		(4.6)
Middle Market Commercial [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	12.2
Catastrophe [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	$ 3.5	$ (2.0)	$ 1.4