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, 2019	Mar. 31, 2018
Insurance [Abstract]
Gross loss and LAE reserves, beginning of period	$ 5,304.1	$ 5,058.5
Reinsurance recoverable on unpaid losses, beginning of period	1,472.6	1,455.0
Net loss and LAE reserves, beginning of period	3,831.5	3,603.5
Net incurred losses and LAE in respect of losses occurring in current year	712.8	677.4
Net incurred losses and LAE in respect of losses occurring in prior years	(13.2)	(6.5)
Total incurred losses and LAE	699.6	670.9
Net payments of losses and LAE in respect of losses occurring in Current year	175.3	181.0
Net payments of losses and LAE in respect of losses occurring in Prior years	459.1	425.5
Total payments	634.4	606.5
Net reserve for losses and LAE, end of period	3,896.7	3,667.9
Reinsurance recoverable on unpaid losses, end of Period	1,484.9	1,420.0
Gross reserve for losses and LAE, end of period	$ 5,381.6	$ 5,087.9