Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2021 USD ($) Integer	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle
Number of Operating Segments \| Integer	3
Ceded Policy Loans	$ 3,373.7	$ 3,390.6
Premium Receivable, Allowance for Credit Loss	34.2	38.8
Premiums Receivable, Gross	530.7	525.8
Premium Receivable, Credit Loss Expense (Reversal)	(4.6)	15.0
Accumulated Depreciation for Property and Equipment	1,259.6	1,239.9
Value of Business Acquired	73.1	83.8
Accumulated Amortization of Value of Business Acquired	157.7	153.7
Amortization of Value of Business Acquired	5.7	6.1	$ 7.1
Prepaid Reinsurance Premiums	777.1	813.0
Deferred Gain on Reinsurance	3.7	5.6
Reinsurance Recoverable, Allowance for Credit Loss	2.3	11.7
Reinsurance Recoverable, Credit Loss Expense (Reversal)	(9.4)	9.9
Premium Tax Expense	166.0	175.5	170.1
Participation Fund Account Assets (PFA)	297.7	319.8
Change in Insurance Reserves and Liabilities	(74.2)	(1,481.8)	(241.1)
Deferred Policy Acquisition Cost, Experience Adjustment	$ 15.1
Previously Reported
New Accounting Pronouncements or Change in Accounting Principle
Change in Insurance Reserves and Liabilities		$ 128.2	$ 135.0

Seperate fund for mutual policies in-force.

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Loans ceded to reinsurers. These loans use insurance policy funds such as cash surrender values as collateral.

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Taxes incurred for premium written during the period.

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Accumulated amortization of other deferred costs capitalized at the end of the reporting period. Does not include deferred finance costs, deferred acquisition costs of insurance companies, or deferred leasing costs for real estate operations.

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Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Amount of expense for writeoff from unexpected insurance contract termination, charged against previously capitalized deferred policy acquisition cost.

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Total carrying amount of consideration received or receivable as of the balance sheet date representing potential earnings that were not as yet recognized as revenue or other forms of income in conformity with GAAP.

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The increase (decrease) in insurance liability balances during the period.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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Amount of expense (reversal of expense) for expected credit loss on premium receivable.

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The carrying value as of the balance sheet date of the estimated valuation allowance to reduce gross premiums receivable to net realizable value.

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Gross amount, as of the balance sheet date, due the entity from (a) agents and insureds, (b) uncollected premiums and (c) others, before deducting the allowance for doubtful accounts.

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The unexpired portion of premiums ceded on policies in force as of the balance sheet date.

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Amount of amortization expense (reversal of expense) for present value of future profits of insurance contract acquired in business combination.

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Amount of expense (reversal of expense) for expected credit loss on reinsurance recoverable.

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Amount of allowance for credit loss on reinsurance recoverable.

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Amount, after accumulated amortization, of present value of future profits of insurance contract acquired in business combination.

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