Reserve For Losses And Loss Expenses (Schedule Of Reconciliation Of Unpaid Losses And Loss Expenses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Business Acquisition [Line Items]
Gross liability at beginning of year	$ 6,623.2	$ 6,363.9	$ 6,456.2	$ 5,881.2
Reinsurance recoverable at beginning of year	(1,516.7)	(1,433.1)	(1,480.0)	(1,340.3)
Net liability at beginning of year	5,106.5	4,930.8	4,976.2	4,540.9
Acquisition of net reserves for losses and loss expenses	0.0	0.0	0.0	257.0
Net losses incurred related to:
Current year	406.0	425.5	1,207.0	1,267.7
Prior years	(29.7)	(8.6)	(92.9)	(94.1)
Total incurred	376.3	416.9	1,114.1	1,173.6
Net paid losses related to:
Current year	70.6	60.8	111.8	95.4
Prior years	301.1	287.8	864.5	866.8
Total paid	371.7	348.6	976.3	962.2
Foreign exchange revaluation	4.0	(12.3)	1.1	(22.5)
Net liability at end of year	5,115.1	4,986.8	5,115.1	4,986.8
Reinsurance recoverable at end of year	1,550.7	1,449.8	1,550.7	1,449.8
Gross liability at end of year	$ 6,665.8	$ 6,436.6	$ 6,665.8	6,436.6
Hong Kong And Singapore Operations Of Royal Sun Alliance Insurance Plc [Member]
Business Acquisition [Line Items]
Acquisition of net reserves for losses and loss expenses				252.8
Labuan Operations of Royal Sun Alliance Insurance Plc [Member]
Business Acquisition [Line Items]
Acquisition of net reserves for losses and loss expenses				$ 4.2