Investments Components of Net Investment Income (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 478.9	$ 454.6	$ 408.4
Investment expenses	(22.4)	(22.8)	(18.9)
Net investment income	456.5	431.8	389.5
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	353.4	357.7	321.9
Fixed maturities \| U.S. government obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	18.2	28.3	46.2
Fixed maturities \| State and local government obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	52.3	60.7	50.1
Fixed maturities \| Foreign government obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	0.4	0.4	0.4
Fixed maturities \| Corporate debt securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	110.7	102.4	82.1
Fixed maturities \| Residential mortgage-backed securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	46.1	52.2	44.9
Fixed maturities \| Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	1.2	2.1	0.0
Fixed maturities \| Commercial mortgage-backed securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	81.6	74.6	66.0
Fixed maturities \| Other asset-backed securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	28.0	22.0	16.7
Fixed maturities \| Redeemable preferred stocks
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	14.9	15.0	15.5
Equity securities \| Nonredeemable preferred stocks
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	48.6	43.7	38.6
Equity securities \| Common equities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	57.2	51.0	46.6
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 19.7	$ 2.2	$ 1.3

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)
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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)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 944
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+ 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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-Section S99
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