Reserve For Losses And Loss Expenses (Schedule Of Reserve For Losses And Loss Expenses) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Insurance Loss Reserves [Abstract]
Outstanding loss reserves	$ 1,807.5	$ 1,678.5
Reserves for losses incurred but not reported	4,831.7	4,777.7
Reserve for losses and loss expenses	$ 6,639.2	$ 6,456.2	$ 5,881.2	$ 5,766.5