Losses and Loss Expenses Payable Losses and Loss Expenses Payable - Schedule of Activity in Liability for Unpaid Losses and Loss Expenses Payable (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Abstract]
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability	$ 1,086.6	$ 1,128.9	$ 1,066.5	$ 1,146.8
Reinsurance Recoverables, Including Reinsurance Premium Paid	39.7	4.6	13.6	5.5
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,046.9	1,124.3	$ 1,052.9	$ 1,141.3
Current Year Claims and Claims Adjustment Expense	246.9	215.7
Prior Year Claims and Claims Adjustment Expense	(7.5)	(21.4)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	239.4	194.3
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	74.1	59.3
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	171.3	152.0
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	$ 245.4	$ 211.3