Investments - Components of Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Net Investment Income [Line Items]
Investment income	$ 19.2	$ 19.6
Investment expenses	0.3	0.2
Net investment income	18.9	19.4
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income	3.1	2.8
Cash and Cash Equivalents, and Other [Member]
Net Investment Income [Line Items]
Investment income	1.0	1.7
Fixed Maturities [Member]
Net Investment Income [Line Items]
Investment income	$ 15.1	$ 15.1

Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(b))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

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-Section 50
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Amount of expenses related to the generation of investment income.

+ References

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-SubTopic 10
-Section S99
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

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