Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Insurance [Abstract]
Gross reserve for losses and LAE, beginning of period	$ 6,024.0	$ 5,654.4
Reinsurance recoverable on unpaid losses, beginning of period	1,641.6	1,574.8
Net reserve for losses and LAE, beginning of period	4,382.4	4,079.6
Net incurred losses and LAE in respect of losses occurring in current year	789.5	730.2
Net incurred losses and LAE in respect of losses occurring in prior years	(8.2)	(2.0)
Total incurred losses and LAE	781.3	728.2
Net payments of losses and LAE in respect of losses occurring in current year	178.5	174.6
Net payments of losses and LAE in respect of losses occurring in prior years	435.0	484.4
Total payments	613.5	659.0
Net reserve for losses and LAE, end of period	4,550.2	4,148.8
Reinsurance recoverable on unpaid losses, end of period	1,673.5	1,576.1
Gross reserve for losses and LAE, end of period	$ 6,223.7	$ 5,724.9