Reserve For Losses And Loss Expenses (Schedule Of Reconciliation Of Unpaid Losses And Loss Expenses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Insurance Loss Reserves [Abstract]
Gross liability at beginning of year	$ 6,639.2	$ 6,456.2
Reinsurance recoverable at beginning of year	(1,625.0)	(1,480.0)
Net liability at beginning of year	5,014.2	4,976.2
Net losses incurred related to:
Current year	360.5	397.8
Prior years	(1.5)	(25.4)
Total incurred	359.0	372.4
Net paid losses related to:
Current year	6.5	6.7
Prior years	336.9	282.5
Total paid	343.4	289.2
Foreign exchange revaluation	7.3	3.7
Net liability at end of year	5,037.1	5,063.1
Reinsurance recoverable at end of year	1,725.6	1,512.0
Gross liability at end of year	$ 6,762.7	$ 6,575.1