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Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
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United States Government and Government Agencies and Authorities |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | $ 22.4 | $ 43.7
Gross Unrealized Loss, Less than 12 Months |  | 0.3 | 4.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 233.6 | 201.3
Gross Unrealized Loss, Greater than 12 Months |  | 17.1 | 23.9
States, Municipalities, and Political Subdivisions |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 168.2 | 425.8
Gross Unrealized Loss, Less than 12 Months |  | 5.8 | 15.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 1,902.4 | 1,926.2
Gross Unrealized Loss, Greater than 12 Months |  | 474.4 | 554.4
Foreign Governments |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 202.5 | 171.9
Gross Unrealized Loss, Less than 12 Months |  | 7.9 | 10.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 324.1 | 266.3
Gross Unrealized Loss, Greater than 12 Months |  | 171.5 | 142.9
Public Utilities |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 618.8 | 1,281.7
Gross Unrealized Loss, Less than 12 Months |  | 20.0 | 48.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 1,540.9 | 1,549.5
Gross Unrealized Loss, Greater than 12 Months |  | 262.2 | 316.0
Mortgage/Asset-backed Securities |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months | [1] | 56.8 | 199.9
Gross Unrealized Loss, Less than 12 Months | [1] | 0.4 | 8.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | [1] | 281.7 | 285.9
Gross Unrealized Loss, Greater than 12 Months | [1] | 16.9 | 28.3
All Other Corporate Bonds |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 1,776.7 | 4,904.4
Gross Unrealized Loss, Less than 12 Months |  | 47.9 | 182.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 11,260.3 | 12,209.3
Gross Unrealized Loss, Greater than 12 Months |  | 1,488.7 | 1,978.3
Redeemable Preferred Stocks |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 0.0 | 3.9
Gross Unrealized Loss, Less than 12 Months |  | 0.0 | 0.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 3.7 | 3.7
Gross Unrealized Loss, Greater than 12 Months |  | 0.2 | 0.3
Fixed Maturity Securities |  |  |
Debt Securities, Available-for-sale, Unrealized Loss Position |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 2,845.4 | 7,031.3
Gross Unrealized Loss, Less than 12 Months |  | 82.3 | 269.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 15,546.7 | 16,442.2
Gross Unrealized Loss, Greater than 12 Months |  | $ 2,431.0 | $ 3,044.1
[1] | 1 Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types

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