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Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Schedule Of Available For Sale Securities [Line Items] |  |
Debt Securities, Amortized cost | $ 7,256.7 | $ 7,895.2
Debt securities, Estimated fair value | 7,015.3 | 7,157.5
U.S. Treasury bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost, Due in one year or less | 40.6 |
Amortized cost, Due after one through five years | 133.8 |
Amortized cost, Due after five through ten years | 4.7 |
Amortized cost, Due after ten years | 12.7 |
Debt Securities, Amortized cost | 191.8 | 203.3
Estimated fair value, Due in one year or less | 40.4 |
Estimated fair value, Due after one through five years | 132.7 |
Estimated fair value, Due after five through ten years | 4.9 |
Estimated fair value, Due after ten years | 12.4 |
Debt securities, Estimated fair value | 190.4 | 199.3
Municipal 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 | 52.2 |
Amortized cost, Due after five through ten years | 231.6 |
Amortized cost, Due after ten years | 484.8 |
Debt Securities, Amortized cost | 777.2 | 1,373.7
Estimated fair value, Due in one year or less | 8.6 |
Estimated fair value, Due after one through five years | 50.9 |
Estimated fair value, Due after five through ten years | 207.7 |
Estimated fair value, Due after ten years | 464.2 |
Debt securities, Estimated fair value | 731.4 | 1,245.8
Foreign government bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost, Due in one year or less | 17.0 |
Amortized cost, Due after one through five years | 136.7 |
Amortized cost, Due after five through ten years | 70.3 |
Amortized cost, Due after ten years | 6.7 |
Debt Securities, Amortized cost | 230.7 | 228.4
Estimated fair value, Due in one year or less | 16.9 |
Estimated fair value, Due after one through five years | 138.8 |
Estimated fair value, Due after five through ten years | 64.8 |
Estimated fair value, Due after ten years | 6.1 |
Debt securities, Estimated fair value | 226.6 | 219.3
Governmental agency bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost, Due in one year or less | 8.5 |
Amortized cost, Due after one through five years | 76.4 |
Amortized cost, Due after five through ten years | 99.8 |
Amortized cost, Due after ten years | 44.9 |
Debt Securities, Amortized cost | 229.6 | 207.7
Estimated fair value, Due in one year or less | 8.5 |
Estimated fair value, Due after one through five years | 76.4 |
Estimated fair value, Due after five through ten years | 99.3 |
Estimated fair value, Due after ten years | 37.8 |
Debt securities, Estimated fair value | 222.0 | 195.4
U.S. corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost, Due in one year or less | 13.6 |
Amortized cost, Due after one through five years | 495.3 |
Amortized cost, Due after five through ten years | 272.0 |
Amortized cost, Due after ten years | 84.3 |
Debt Securities, Amortized cost | 865.2 | 1,007.0
Estimated fair value, Due in one year or less | 13.6 |
Estimated fair value, Due after one through five years | 497.7 |
Estimated fair value, Due after five through ten years | 275.0 |
Estimated fair value, Due after ten years | 78.2 |
Debt securities, Estimated fair value | 864.5 | 958.4
Foreign corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost, Due in one year or less | 34.2 |
Amortized cost, Due after one through five years | 299.7 |
Amortized cost, Due after five through ten years | 129.7 |
Amortized cost, Due after ten years | 29.8 |
Debt Securities, Amortized cost | 493.4 | 478.9
Estimated fair value, Due in one year or less | 34.0 |
Estimated fair value, Due after one through five years | 301.9 |
Estimated fair value, Due after five through ten years | 130.8 |
Estimated fair value, Due after ten years | 27.8 |
Debt securities, Estimated fair value | 494.5 | $ 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 | 122.5 |
Amortized cost, Due after one through five years | 1,194.1 |
Amortized cost, Due after five through ten years | 808.1 |
Amortized cost, Due after ten years | 663.2 |
Debt Securities, Amortized cost | 2,787.9 |
Estimated fair value, Due in one year or less | 122.0 |
Estimated fair value, Due after one through five years | 1,198.4 |
Estimated fair value, Due after five through ten years | 782.5 |
Estimated fair value, Due after ten years | 626.5 |
Debt securities, Estimated fair value | 2,729.4 |
Total Mortgage-Backed Securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized cost | 4,468.8 |
Estimated fair value | $ 4,285.9 |

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