Reserve For Losses And Loss Expenses (Schedule Of Reserve For Losses And Loss Expenses) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Insurance Loss Reserves [Abstract]
Outstanding loss reserves	$ 1,751.0		$ 1,678.5
Reserves for losses incurred but not reported	4,914.8		4,777.7
Reserve for losses and loss expenses	$ 6,665.8	$ 6,623.2	$ 6,456.2	$ 6,436.6	$ 6,363.9	$ 5,881.2