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Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Schedule Of Available For Sale Securities [Line Items] |  |
Debt Securities, Amortized cost | $ 7,730.9 | $ 7,895.2
Debt securities, Estimated fair value | 7,265.9 | 7,157.5
U.S. Treasury bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost, Due in one year or less | 32.9 |
Amortized cost, Due after one through five years | 117.6 |
Amortized cost, Due after five through ten years | 4.7 |
Amortized cost, Due after ten years | 25.5 |
Debt Securities, Amortized cost | 180.7 | 203.3
Estimated fair value, Due in one year or less | 32.8 |
Estimated fair value, Due after one through five years | 114.7 |
Estimated fair value, Due after five through ten years | 4.6 |
Estimated fair value, Due after ten years | 23.5 |
Debt securities, Estimated fair value | 175.6 | 199.3
Municipal bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost, Due in one year or less | 7.1 |
Amortized cost, Due after one through five years | 83.0 |
Amortized cost, Due after five through ten years | 283.1 |
Amortized cost, Due after ten years | 471.7 |
Debt Securities, Amortized cost | 844.9 | 1,373.7
Estimated fair value, Due in one year or less | 7.1 |
Estimated fair value, Due after one through five years | 80.5 |
Estimated fair value, Due after five through ten years | 247.6 |
Estimated fair value, Due after ten years | 441.3 |
Debt securities, Estimated fair value | 776.5 | 1,245.8
Foreign government bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost, Due in one year or less | 13.0 |
Amortized cost, Due after one through five years | 123.7 |
Amortized cost, Due after five through ten years | 73.7 |
Amortized cost, Due after ten years | 6.7 |
Debt Securities, Amortized cost | 217.1 | 228.4
Estimated fair value, Due in one year or less | 13.0 |
Estimated fair value, Due after one through five years | 125.1 |
Estimated fair value, Due after five through ten years | 67.9 |
Estimated fair value, Due after ten years | 5.6 |
Debt securities, Estimated fair value | 211.6 | 219.3
Governmental agency bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost, Due in one year or less | 8.6 |
Amortized cost, Due after one through five years | 1.4 |
Amortized cost, Due after five through ten years | 149.5 |
Amortized cost, Due after ten years | 44.3 |
Debt Securities, Amortized cost | 203.8 | 207.7
Estimated fair value, Due in one year or less | 8.6 |
Estimated fair value, Due after one through five years | 1.4 |
Estimated fair value, Due after five through ten years | 144.2 |
Estimated fair value, Due after ten years | 35.6 |
Debt securities, Estimated fair value | 189.8 | 195.4
U.S. corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost, Due in one year or less | 12.4 |
Amortized cost, Due after one through five years | 545.2 |
Amortized cost, Due after five through ten years | 295.6 |
Amortized cost, Due after ten years | 95.2 |
Debt Securities, Amortized cost | 948.4 | 1,007.0
Estimated fair value, Due in one year or less | 12.5 |
Estimated fair value, Due after one through five years | 539.0 |
Estimated fair value, Due after five through ten years | 290.5 |
Estimated fair value, Due after ten years | 83.6 |
Debt securities, Estimated fair value | 925.6 | 958.4
Foreign corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost, Due in one year or less | 22.1 |
Amortized cost, Due after one through five years | 309.9 |
Amortized cost, Due after five through ten years | 128.4 |
Amortized cost, Due after ten years | 31.2 |
Debt Securities, Amortized cost | 491.6 | 478.9
Estimated fair value, Due in one year or less | 22.0 |
Estimated fair value, Due after one through five years | 308.4 |
Estimated fair value, Due after five through ten years | 126.1 |
Estimated fair value, Due after ten years | 28.0 |
Debt securities, Estimated fair value | 484.5 | $ 463.6
Aggregate Debt Securities Excluding Mortgage Backed And Asset Backed Securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost, Due in one year or less | 96.1 |
Amortized cost, Due after one through five years | 1,180.8 |
Amortized cost, Due after five through ten years | 935.0 |
Amortized cost, Due after ten years | 674.6 |
Debt Securities, Amortized cost | 2,886.5 |
Estimated fair value, Due in one year or less | 96.0 |
Estimated fair value, Due after one through five years | 1,169.1 |
Estimated fair value, Due after five through ten years | 880.9 |
Estimated fair value, Due after ten years | 617.6 |
Debt securities, Estimated fair value | 2,763.6 |
Total Mortgage-Backed Securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost | 4,844.4 |
Estimated fair value | $ 4,502.3 |

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