Reserve For Losses And Loss Expenses (Schedule Of Reconciliation Of Unpaid Losses And Loss Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Gross liability at beginning of year	$ 6,456.2	$ 5,881.2	$ 5,766.5
Reinsurance recoverable at beginning of year	(1,480.0)	(1,340.3)	(1,234.5)
Net liability at beginning of year	4,976.2	4,540.9	4,532.0
Acquisition of net reserves for losses and loss expenses	0.0	259.3	0.0
Net losses incurred related to:
Current year	1,600.1	1,667.9	1,411.8
Prior years	(98.3)	(81.6)	(212.6)
Total incurred	1,501.8	1,586.3	1,199.2
Net paid losses related to:
Current year	208.4	186.0	171.8
Prior years	1,238.6	1,201.6	1,001.5
Total paid	1,447.0	1,387.6	1,173.3
Foreign exchange revaluation	(16.8)	(22.7)	(17.0)
Net liability at end of year	5,014.2	4,976.2	4,540.9
Reinsurance recoverable at end of year	1,625.0	1,480.0	1,340.3
Gross liability at end of year	$ 6,639.2	6,456.2	$ 5,881.2
Hong Kong And Singapore Operations Of Royal Sun Alliance Insurance Plc
Business Acquisition [Line Items]
Acquisition of net reserves for losses and loss expenses		255.2
Labuan Operations of Royal Sun Alliance
Business Acquisition [Line Items]
Acquisition of net reserves for losses and loss expenses		$ 4.1