Liability for Unpaid Claims and Claims Adjustment Expense (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense
Balance at January 1	$ 23,076.7	$ 23,149.0	$ 23,222.0
Less Reinsurance Recoverable	2,246.8	2,227.3	2,182.0
Net Balance at January 1	20,829.9	20,921.7	21,040.0
Incurred Related to
Current Year	6,327.8	6,113.2	5,832.3
Paid Related to
Current Year	(2,727.0)	(2,532.4)	(2,354.7)
Prior Years	(4,430.3)	(4,511.0)	(4,428.8)
Total Paid	(7,157.3)	(7,043.4)	(6,783.5)
Reserves Ceded Pursuant to Reinsurance Transaction	(6,141.5)	0.0	0.0
Net Balance at December 31	15,801.3	20,829.9	20,921.7
Plus Reinsurance Recoverable	8,378.9	2,246.8	2,227.3
Balance at December 31	24,180.2	23,076.7	23,149.0
Amount Related to Interest
Incurred Related to
Prior Years	997.8	1,036.5	1,049.8
Incurred Claims
Incurred Related to
Prior Years	878.7	(274.1)	(106.2)
Foreign Currency
Incurred Related to
Prior Years	65.9	76.0	(110.7)
Operating Expense
Incurred Related to
Total Incurred	$ 8,270.2	$ 6,951.6	$ 6,665.2

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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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Amount, after effects of reinsurance, of payments to settle claims incurred in the current period and related claims settlement costs.

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Amount, after effects of reinsurance, of payments to settle claims incurred in prior periods and related claims settlement costs.

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Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred and costs incurred in the claim settlement process.

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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 effects of reinsurance, of payments to settle insured claims and pay costs incurred in the claims settlement process.

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Estimated amount after valuation allowance of reinsurance recoverables, due from reinsurers for incurred claims not yet reported, and claims settlement expenses.

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Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period and related claims settlement costs.

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Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

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