Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($) Integer	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2020 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of Operating Segments \| Integer	3
Ceded Policy Loans	$ 3,390.6	$ 3,490.6
Premium Receivable, Allowance for Credit Loss	38.8			$ 23.8
Premiums Receivable, Gross	525.8			543.0
Premium Receivable, Credit Loss Expense (Reversal)	15.0
Accumulated Depreciation for Property and Equipment	1,239.9	1,195.3
Value of Business Acquired	83.8	88.7
Accumulated Amortization of Value of Business Acquired	153.7	144.1
Amortization of Value of Business Acquired	6.1	7.1	$ 6.4
Prepaid Reinsurance Premiums	813.0
Deferred Gain on Reinsurance	5.6	8.5
Reinsurance Recoverable, Allowance for Credit Loss	11.7	0.0		$ 1.8
Reinsurance Recoverable, Credit Loss Expense (Reversal)	9.9
Premium Tax Expense	175.5	170.1	$ 155.8
Participation Fund Account Assets (PFA)	$ 319.8	$ 311.2

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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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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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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