Losses and Loss Expenses Payable Losses and Loss Expenses Payable - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Document Period End Date	Mar. 31, 2020
Prior Year Claims and Claims Adjustment Expense	$ (7.5)	$ (21.4)
Specialty Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(1.3)
Homeowners [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(2.2)	(0.6)
Middle Market Commercial [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	5.1	3.1
Personal Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(5.2)	6.8
Workers' compensation [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	4.0	4.5
Small commercial [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	5.4	4.8
Personal Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(6.4)
Total Insurance Segments [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 10.5	$ 20.9