Schedule VI - Supplemental Information Concerning Property-Casualty Insurance Operations (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplemental Information for Property, Casualty Insurance Underwriters
Current Year Claims and Claims Adjustment Expenses		$ 2,557.8	$ 2,577.6	$ 2,520.7
Prior Year Claims and Claims Adjustment Expenses		(83.8)	(30.9)	(77.8)
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters, Consolidated Property and Casualty Insurance Entity [Member]
Supplemental Information for Property, Casualty Insurance Underwriters
Deferred Policy Acquisition Costs	[1]	327.2	324.0	314.1
Reserves for Unpaid Claims and Claims Adjustment Expenses	[1],[2]	6,328.0	6,021.3	5,756.0
Discount If Any Deducted From Reserves	[1]	196.9	209.6	216.5
Unearned Premiums	[1],[2]	1,765.2	1,733.0	1,696.4
Earned Premiums	[1]	3,394.2	3,433.3	3,278.6
Net Investment Income	[1]	352.2	356.7	341.9
Current Year Claims and Claims Adjustment Expenses	[1]	2,380.5	2,424.1	2,350.2
Prior Year Claims and Claims Adjustment Expenses	[1]	(27.4)	12.4	(6.4)
Amortization of Deferred Policy Acquisition Costs	[1]	500.9	531.5	488.4
Paid Claims and Claims Adjustment Expenses	[1]	2,046.3	2,171.3	2,083.0
Premiums Written	[1]	$ 3,431.3	$ 3,469.3	$ 3,382.0

[1] Includes consolidated property-casualty entities. The amounts relating to the Company's unconsolidated property-casualty subsidiaries and the proportionate share of the registrant's and its subsidiaries' 50%-or-less owned property-casualty equity investees are immaterial and have, therefore, been omitted from this schedule.

[2] See note (2) to Schedule III.