Reserve For Losses And Loss Expenses (Schedule Of Reconciliation Of Unpaid Losses And Loss Expenses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Insurance Loss Reserves [Abstract]
Gross liability at beginning of year	$ 6,575.1	$ 5,905.1	$ 6,456.2	$ 5,881.2
Reinsurance recoverable at beginning of year	(1,512.0)	(1,350.3)	(1,480.0)	(1,340.3)
Net liability at beginning of year	5,063.1	4,554.8	4,976.2	4,540.9
Acquisition of net reserve for losses and loss expenses	0.0	257.0	0.0	257.0
Net losses incurred related to:
Current year	403.3	453.3	801.1	842.2
Prior years	(37.9)	(21.8)	(63.3)	(85.5)
Total incurred	365.4	431.5	737.8	756.7
Net paid losses related to:
Current year	34.5	28.0	41.2	34.6
Prior years	280.9	285.8	563.4	578.9
Total paid	315.4	313.8	604.6	613.5
Foreign exchange revaluation	(6.6)	1.3	(2.9)	(10.3)
Net liability at end of year	5,106.5	4,930.8	5,106.5	4,930.8
Reinsurance recoverable at end of year	1,516.7	1,433.1	1,516.7	1,433.1
Gross liability at end of year	6,623.2	6,363.9	6,623.2	6,363.9
Business Acquisition [Line Items]
Acquisition of net reserve for losses and loss expenses	$ 0.0	257.0	$ 0.0	257.0
Hong Kong And Singapore Operations Of Royal Sun Alliance Insurance Plc [Member]
Insurance Loss Reserves [Abstract]
Acquisition of net reserve for losses and loss expenses		252.8		252.8
Business Acquisition [Line Items]
Acquisition of net reserve for losses and loss expenses		252.8		252.8
Royal and Sun Alliance Labuan branch
Insurance Loss Reserves [Abstract]
Acquisition of net reserve for losses and loss expenses		4.2		4.2
Business Acquisition [Line Items]
Acquisition of net reserve for losses and loss expenses		$ 4.2		$ 4.2