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Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions | Dec. 31, 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 |  | $ 21.8 | $ 43.7
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 0.4 | 4.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 235.6 | 201.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 15.6 | 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 |  | 138.2 | 425.8
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 3.6 | 15.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 1,922.2 | 1,926.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 462.8 | 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 |  | 264.6 | 171.9
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 5.5 | 10.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 318.2 | 266.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 151.8 | 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 |  | 642.1 | 1,281.7
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 19.4 | 48.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 1,710.3 | 1,549.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 270.7 | 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] | 206.6 | 199.9
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | [1] | 0.4 | 8.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer | [1] | 275.3 | 285.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | [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 |  | 2,060.0 | 4,904.4
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 40.6 | 182.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 11,087.9 | 12,209.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 1,518.9 | 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
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 0.0 | 0.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 3.9 | 3.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 0.1 | 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 |  | 3,333.3 | 7,031.3
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 69.9 | 269.9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 15,553.4 | 16,442.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | $ 2,436.8 | $ 3,044.1
[1] | 2 Includes credit-tranched securities collateralized by loan obligations, auto loans, and other asset types

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