Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Insurance [Abstract]
Gross loss and LAE reserves, beginning of period	$ 6,949.4	$ 6,574.4
Reinsurance recoverable on unpaid losses, beginning of period	2,274.8	2,280.8
Net loss and LAE reserves, beginning of period	4,674.6	4,293.6
Net incurred losses and LAE in respect of losses occurring in current year	2,414.1	2,145.1
Net incurred losses and LAE in respect of losses occurring in prior years	(44.6)	(23.8)
Total incurred losses and LAE	2,369.5	2,121.3
Net payments of losses and LAE in respect of losses occurring in Current year	851.2	775.1
Net payments of losses and LAE in respect of losses occurring in Prior years	1,131.5	1,088.3
Total payments	1,982.7	1,863.4
Effect of foreign exchange rate changes	40.3	(52.6)
Net reserve for losses and LAE, end of period	5,101.7	4,498.9
Reinsurance recoverable on unpaid losses, end of Period	2,533.7	2,303.4
Gross reserve for losses and LAE, end of period	$ 7,635.4	$ 6,802.3