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Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | $ 1,843.8 | $ 552.0
Unrealized losses, Less than 12 months | (11.5) | (8.2)
Estimated fair value, 12 months or longer | 2,332.7 | 5,327.6
Unrealized losses, 12 months or longer | (291.8) | (759.1)
Estimated fair value, Total | 4,176.5 | 5,879.6
Unrealized losses, Total | (303.3) | (767.3)
U.S. Treasury bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 1.3 | 8.2
Unrealized losses, Less than 12 months | 0.0 | (0.1)
Estimated fair value, 12 months or longer | 57.9 | 55.4
Unrealized losses, 12 months or longer | (3.0) | (4.4)
Estimated fair value, Total | 59.2 | 63.6
Unrealized losses, Total | (3.0) | (4.5)
Municipal bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 49.7 | 107.4
Unrealized losses, Less than 12 months | (0.2) | (0.9)
Estimated fair value, 12 months or longer | 455.5 | 956.8
Unrealized losses, 12 months or longer | (55.1) | (135.8)
Estimated fair value, Total | 505.2 | 1,064.2
Unrealized losses, Total | (55.3) | (136.7)
Foreign government bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 10.4 | 33.3
Unrealized losses, Less than 12 months | 0.0 | (0.1)
Estimated fair value, 12 months or longer | 78.2 | 101.4
Unrealized losses, 12 months or longer | (6.6) | (10.4)
Estimated fair value, Total | 88.6 | 134.7
Unrealized losses, Total | (6.6) | (10.5)
Governmental agency bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 172.8 | 0.4
Unrealized losses, Less than 12 months | (0.4) | 0.0
Estimated fair value, 12 months or longer | 40.4 | 118.9
Unrealized losses, 12 months or longer | (7.3) | (12.5)
Estimated fair value, Total | 213.2 | 119.3
Unrealized losses, Total | (7.7) | (12.5)
Governmental agency mortgage-backed securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 1,351.7 | 338.3
Unrealized losses, Less than 12 months | (9.7) | (6.6)
Estimated fair value, 12 months or longer | 1,419.2 | 3,225.3
Unrealized losses, 12 months or longer | (196.2) | (520.2)
Estimated fair value, Total | 2,770.9 | 3,563.6
Unrealized losses, Total | (205.9) | (526.8)
U.S. corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 189.5 | 45.1
Unrealized losses, Less than 12 months | (0.7) | (0.4)
Estimated fair value, 12 months or longer | 140.9 | 602.5
Unrealized losses, 12 months or longer | (15.4) | (54.8)
Estimated fair value, Total | 330.4 | 647.6
Unrealized losses, Total | (16.1) | (55.2)
Foreign corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 68.4 | 19.3
Unrealized losses, Less than 12 months | (0.5) | (0.1)
Estimated fair value, 12 months or longer | 140.6 | 267.3
Unrealized losses, 12 months or longer | (8.2) | (21.0)
Estimated fair value, Total | 209.0 | 286.6
Unrealized losses, Total | $ (8.7) | $ (21.1)

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