Statutory Restrictions on Investments and Stockholders' Equity (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Statutory Accounting Practices [Line Items]
Investments on deposit with state treasurers	$ 127.2	$ 124.1
Dividends available to parent from subsidiaries	535.0
Loans and advances available to parent from subsidiaries	114.2
Differences in state prescribed or permitted practices to NAIC Statutory Accounting	354.6	347.1
FATICO
Statutory Accounting Practices [Line Items]
Statutory surplus maintained by insurance subsidiary	1,600.0	1,500.0
Statutory net income of insurance subsidiary	$ 182.8	$ 198.3	$ 436.3

Statutory accounting practices statutory amount available for loans and advances without regulatory approval.

No definition available.

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- Definition

Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 825
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477351/944-825-50-1A

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- Definition

Amount of an item which resulted in a difference from applying state prescribed or permitted statutory accounting practices rather than statutory accounting practices prescribed by National Association of Insurance Commissioners.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477908/944-505-50-3

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

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- Definition

Amount of dividends that can be declared or paid without approval of regulatory agency.

+ References

No definition available.

+ Details

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- Definition

The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477908/944-505-50-1

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- Definition

Amount of net income for the period determined using accounting principles prescribed or permitted by insurance regulators.

+ References

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