Investments Components of Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 408.4	$ 422.0	$ 443.0
Investment expenses	(18.9)	(18.8)	(15.4)
Net investment income	389.5us-gaap_NetInvestmentIncome	403.2us-gaap_NetInvestmentIncome	427.6us-gaap_NetInvestmentIncome
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	321.9	338.0	351.2
Fixed maturities \| U.S. government obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	46.2	50.2	49.8
Fixed maturities \| State and local government obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	50.1	48.0	51.1
Fixed maturities \| Foreign government obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	0.4	0.2	0
Fixed maturities \| Corporate debt securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	82.1	98.8	107.5
Fixed maturities \| Residential mortgage-backed securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	44.9	28.1	16.1
Fixed maturities \| Commercial mortgage-backed securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	66.0	74.8	82.2
Fixed maturities \| Other asset-backed securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	16.7	16.7	20.3
Fixed maturities \| Redeemable preferred stocks
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	15.5	21.2	24.2
Equity securities \| Nonredeemable preferred stocks
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	38.6	36.2	43.8
Equity securities \| Common equities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	46.6	45.8	44.9
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 1.3	$ 2.0	$ 3.1

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.

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1, 2 , 3, 4, 5

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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 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.2)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: 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)

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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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 16

-Article 12

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)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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