Losses and Loss Expenses Payable Losses and Loss Expenses Payable - Additional Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)
Liability for Claims and Claims Adjustment Expense [Line Items]
Document Period End Date	Mar. 31, 2018
Prior Year Claims and Claims Adjustment Expense	$ 20.4	$ 9.7
Unallocated loss adjustment expense [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	3.8	$ (2.7)
Business Insurance Segments [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(16.3)
Specialty Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(0.3)
Homeowners [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Number of reported claims		3
Prior Year Claims and Claims Adjustment Expense	2.8	$ (2.8)
Middle Market Commercial [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	5.5
Personal Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	4.0
Commercial Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	3.5
Workers' compensation [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	2.3	1.9
E&S Casualty [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(1.9)
Other Commercial [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		3.2
Small commercial [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		2.0
Personal Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(2.3)
Catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 0.6	$ (2.2)