Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 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	1,603.5	1,516.0
Net incurred losses and LAE in respect of losses occurring in prior years	(34.1)	(24.5)
Total incurred losses and LAE	1,569.4	1,491.5
Net payments of losses and LAE in respect of losses occurring in Current year	504.9	491.2
Net payments of losses and LAE in respect of losses occurring in Prior years	944.1	799.7
Total payments	1,449.0	1,290.9
Effect of foreign exchange rate changes	(14.1)	25.8
Net reserve for losses and LAE, end of period	5,242.9	4,901.0
Reinsurance recoverable on unpaid losses, end of Period	2,263.5	2,261.4
Gross reserve for losses and LAE, end of period	$ 7,506.4	$ 7,162.4