Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Insurance [Abstract]
Gross reserve for losses and LAE, beginning of year	$ 5,654.4	$ 5,304.1	$ 5,058.5
Reinsurance recoverable on unpaid losses, beginning of year	1,574.8	1,472.6	1,455.0
Net loss and LAE reserves, beginning of year	4,079.6	3,831.5	3,603.5
Net incurred losses and LAE in respect of losses occurring in current year	2,877.8	2,893.0	2,733.5
Net incurred losses and LAE in respect of losses occurring in prior years	(32.6)	(28.4)	(8.9)
Total incurred losses and LAE	2,845.2	2,864.6	2,724.6
Net payments of losses and LAE in respect of losses occurring in current year	1,347.7	1,315.4	1,232.3
Net payments of losses and LAE in respect of losses occurring in prior years	1,194.7	1,301.1	1,264.3
Total payments	2,542.4	2,616.5	2,496.6
Net reserve for losses and LAE, end of year	4,382.4	4,079.6	3,831.5
Reinsurance recoverable on unpaid losses, end of year	1,641.6	1,574.8	1,472.6
Gross reserve for losses and LAE, end of year	$ 6,024.0	$ 5,654.4	$ 5,304.1