Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Schedule of Carried Reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	$ 5,304.1	$ 5,058.5	$ 4,660.0	$ 4,201.9
Ceded	(1,472.6)	(1,455.0)
Net	3,831.5	3,603.5	$ 3,310.8	$ 2,906.6
Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	3,596.5	3,439.2
Ceded	(547.4)	(570.2)
Net	3,049.1	2,869.0
Personal Lines [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	1,707.6	1,619.3
Ceded	(925.2)	(884.8)
Net	782.4	734.5
Commercial Multiple Peril Line [Member] | Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	1,206.5	1,104.9
Ceded	(110.8)	(108.9)
Net	1,095.7	996.0
Workers' Compensation Line [Member] | Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	775.9	757.8
Ceded	(183.0)	(180.8)
Net	592.9	577.0
Commercial Automobile Line [Member] | Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	433.5	428.1
Ceded	(25.5)	(27.6)
Net	408.0	400.5
General Liability And Umbrella - Occurrence [Member] | Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	534.1	508.5
Ceded	(133.5)	(126.2)
Net	400.6	382.3
General Liability - Claims Made [Member] | Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	245.9	231.6
Ceded	(16.2)	(17.9)
Net	229.7	213.7
Other Commercial Lines [Member] | Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	400.6	408.3
Ceded	(78.4)	(108.8)
Net	322.2	299.5
Personal Automobile Lines [Member] | Personal Lines [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	1,514.0	1,453.8
Ceded	(917.3)	(878.8)
Net	596.7	575.0
Homeowners [Member] | Personal Lines [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	158.2	133.5
Ceded	(6.0)	(4.4)
Net	152.2	129.1
Other Personal Lines | Personal Lines [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	35.4	32.0
Ceded	(1.9)	(1.6)
Net	$ 33.5	$ 30.4