Reserve For Losses And Loss Expenses (Schedule Of Reserve For Losses And Loss Expenses) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Insurance Loss Reserves [Abstract]
Outstanding loss reserves	$ 1,899.9	$ 1,807.5
Reserves for losses incurred but not reported	4,862.8	4,831.7
Reserve for losses and loss expenses	$ 6,762.7	$ 6,639.2	$ 6,575.1	$ 6,456.2