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, 2018	Mar. 31, 2017
Insurance [Abstract]
Gross loss and LAE reserves, beginning of period	$ 7,745.0	$ 6,949.4
Reinsurance recoverable on unpaid losses, beginning of period	2,608.4	2,274.8
Net loss and LAE reserves, beginning of period	5,136.6	4,674.6
Net incurred losses and LAE in respect of losses occurring in current year	803.7	773.9
Net incurred losses and LAE in respect of losses occurring in prior years	(17.9)	(7.4)
Total incurred losses and LAE	785.8	766.5
Net payments of losses and LAE in respect of losses occurring in Current year	187.2	167.1
Net payments of losses and LAE in respect of losses occurring in Prior years	537.4	452.3
Total payments	724.6	619.4
Effect of foreign exchange rate changes	13.2	7.4
Net reserve for losses and LAE, end of period	5,211.0	4,829.1
Reinsurance recoverable on unpaid losses, end of Period	2,308.2	2,273.9
Gross reserve for losses and LAE, end of period	$ 7,519.2	$ 7,103.0