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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

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- 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 + Details
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- Definition 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16(Column G)) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915 + Details
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