The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

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Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims.

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Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

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Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Carried Reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	$ 6,024.0	$ 5,654.4	$ 5,304.1	$ 5,058.5
Ceded	(1,641.6)	(1,574.8)	(1,472.6)	(1,455.0)
Net	4,382.4	4,079.6	$ 3,831.5	$ 3,603.5
Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	4,116.2	3,815.0
Ceded	(668.7)	(604.9)
Net	3,447.5	3,210.1
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,907.8	1,839.4
Ceded	(972.9)	(969.9)
Net	934.9	869.5
Commercial Multiple Peril Line [Member] \| Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,429.4	1,298.5
Ceded	(163.9)	(126.5)
Net	1,265.5	1,172.0
Workers' Compensation Line [Member] \| Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	795.4	795.3
Ceded	(182.6)	(186.0)
Net	612.8	609.3
Commercial Automobile Line [Member] \| Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	504.1	467.7
Ceded	(30.6)	(27.3)
Net	473.5	440.4
General Liability and Umbrella - Occurrence [Member] \| Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	612.0	575.9
Ceded	(176.0)	(155.5)
Net	436.0	420.4
General Liability - Claims Made [Member] \| Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	323.7	268.1
Ceded	(30.5)	(20.1)
Net	293.2	248.0
Other Commercial Lines [Member] \| Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	451.6	409.5
Ceded	(85.1)	(89.5)
Net	366.5	320.0
Personal Automobile Lines [Member] \| Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,649.9	1,626.4
Ceded	(968.5)	(963.2)
Net	681.4	663.2
Homeowners [Member] \| Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	210.6	172.4
Ceded	(3.0)	(4.8)
Net	207.6	167.6
Other Personal Lines \| Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	47.3	40.6
Ceded	(1.4)	(1.9)
Net	$ 45.9	$ 38.7