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Investments Components of Net Investment Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 563.1	$ 478.9	$ 454.6
Investment expenses	(23.9)	(22.4)	(22.8)
Net investment income	539.2	456.5	431.8
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	422.9	353.4	357.7
Fixed maturities | U.S. government obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	72.7	18.2	28.3
Fixed maturities | State and local government obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	51.5	52.3	60.7
Fixed maturities | Foreign government obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	0.3	0.4	0.4
Fixed maturities | Corporate debt securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	125.2	110.7	102.4
Fixed maturities | Residential mortgage-backed securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	34.7	47.3	54.3
Fixed maturities | Commercial mortgage-backed securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	79.6	81.6	74.6
Fixed maturities | Other asset-backed securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	47.1	28.0	22.0
Fixed maturities | Redeemable preferred stocks
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	11.8	14.9	15.0
Equity securities | Nonredeemable preferred stocks
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	44.1	48.6	43.7
Equity securities | Common equities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	58.3	57.2	51.0
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 37.8	$ 19.7	$ 2.2

X

- Definition

Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.1-5)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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Amount of expenses related to the generation of investment income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.2)
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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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