Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Schedule of (Favorable)/Unfavorable Loss and LAE Reserve Development) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	$ (174.1)	$ (8.9)	$ (0.2)	$ 238.2
Commercial Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(40.9)	(10.8)	219.3
Personal Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		30.8	9.4	10.6
General Liability [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(21.1)	(1.9)	56.0
Commercial Multiple Peril Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(4.0)	2.9	68.8
Workers' Compensation Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(31.0)	(9.1)	(46.7)
Commercial Automobile Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		23.2	2.3	27.5
AIX Program Business [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		17.0	(1.9)	75.1
Surety Product Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(9.0)	0.1	35.3
Umbrella Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		2.3	0.9	(8.9)
Other Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(18.3)	(4.1)	12.2
Other Commercial Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(29.1)	(6.9)	169.7
Other Commercial Lines [Member] | Commercial Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	$ (161.5)
Personal Automobile Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		14.3	3.4	4.8
Homeowners And Other Personal Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		16.5	6.0	5.8
Other Segment [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		$ 1.2	$ 1.2	$ 8.3