Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of (Favorable)/Unfavorable Loss and LAE Reserve Development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	$ (32.6)	$ (28.4)	$ (8.9)
Commercial Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(37.8)	(53.3)	(40.9)
Commercial Lines [Member] | Commercial Multiple Peril Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(0.2)	(19.5)	(4.0)
Commercial Lines [Member] | Workers' Compensation Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(36.0)	(32.6)	(31.0)
Commercial Lines [Member] | Commercial Automobile Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	15.4	5.8	23.2
Commercial Lines [Member] | Hanover Programs Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	10.6	19.8	17.0
Commercial Lines [Member] | Marine Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(17.6)	(13.3)	(10.7)
Commercial Lines [Member] | Surety Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(3.5)	(4.1)	(9.0)
Commercial Lines [Member] | Umbrella Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(1.5)	2.8	2.3
Commercial Lines [Member] | Other Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(5.3)	(4.4)	(0.8)
Commercial Lines [Member] | Other Commercial Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(17.0)	(7.0)	(29.1)
Commercial Lines [Member] | General Liability [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	0.3	(7.8)	(27.9)
Personal Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	1.0	23.7	30.8
Personal Lines [Member] | Personal Automobile Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	5.2	21.6	14.3
Personal Lines [Member] | Homeowners and Other Personal Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(4.2)	2.1	16.5
Other Segment [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	$ 4.2	$ 1.2	$ 1.2