Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) Integer	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of Operating Segments \| Integer	3
Ceded Policy Loans	$ 3,307.5	$ 3,206.1
Accumulated Depreciation for Property and Equipment	1,067.7	989.4
Value of Business Acquired	27.9	31.9
Accumulated Amortization of Value of Business Acquired	130.4	119.2
Amortization of Value of Business Acquired	4.8	5.1	$ 4.6
Deferred Gain on Reinsurance	17.7	23.8
Premium tax expense	158.0	152.5	$ 146.5
Participation Fund Account Assets (PFA)	$ 328.3	$ 331.1

Seperate fund for mutual policies in-force.

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- Definition

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

+ References

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- Definition

Taxes incurred for premium written during the period.

+ References

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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- Definition

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

+ References

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-Name Accounting Standards Codification
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- Definition

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.

+ References

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

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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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