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Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities |  |
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months | $ 6,495.3 | $ 2,037.6
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss | 187.1 | 40.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months | 1,093.2 | 1,144.5
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss | 106.0 | 83.5
United States Government and Government Agencies and Authorities |  |
Schedule of Available-for-sale Securities |  |
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months | 178.4 | 157.9
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss | 8.0 | 3.0
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months | 57.5 | 58.8
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss | 3.1 | 1.8
States, Municipalities, and Political Subdivisions |  |
Schedule of Available-for-sale Securities |  |
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months | 134.3 | 45.7
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss | 1.4 | 0.5
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months | 24.2 | 22.3
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss | 1.1 | 0.7
Foreign Governments |  |
Schedule of Available-for-sale Securities |  |
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months | 38.8 | 13.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss | 0.8 | 0.4
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months | 0.0 | 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss | 0.0 | 0.0
Public Utilities |  |
Schedule of Available-for-sale Securities |  |
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months | 400.7 | 213.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss | 17.8 | 7.9
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months | 152.2 | 133.5
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss | 9.8 | 4.7
Mortgage/Asset-Backed Securities |  |
Schedule of Available-for-sale Securities |  |
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months | 366.3 | 252.5
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss | 9.4 | 1.4
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months | 114.1 | 144.7
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss | 5.3 | 3.3
All Other Corporate Bonds |  |
Schedule of Available-for-sale Securities |  |
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months | 5,376.8 | 1,355.1
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss | 149.7 | 26.8
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months | 745.2 | 785.2
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss | $ 86.7 | $ 73.0

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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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