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, 2023	Mar. 31, 2022
Insurance [Abstract]
Gross reserve for losses and LAE, beginning of period	$ 7,012.6	$ 6,447.6
Reinsurance recoverable on unpaid losses, beginning of period	1,748.6	1,693.8
Net reserve for losses and LAE, beginning of period	5,264.0	4,753.8
Net incurred losses and LAE in respect of losses occurring in current year	1,020.4	793.5
Net incurred losses and LAE in respect of losses occurring in prior years	(3.0)	(6.0)
Total incurred losses and LAE	1,017.4	787.5
Net payments of losses and LAE in respect of losses occurring in current year	222.0	191.1
Net payments of losses and LAE in respect of losses occurring in prior years	667.3	534.2
Total payments	889.3	725.3
Net reserve for losses and LAE, end of period	5,392.1	4,816.0
Reinsurance recoverable on unpaid losses, end of period	1,751.3	1,696.2
Gross reserve for losses and LAE, end of period	$ 7,143.4	$ 6,512.2