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Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions | Jun. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | $ 927.5 | $ 552.0
Unrealized losses, Less than 12 months | (10.2) | (8.2)
Estimated fair value, 12 months or longer | 5,387.4 | 5,327.6
Unrealized losses, 12 months or longer | (817.4) | (759.1)
Estimated fair value, Total | 6,314.9 | 5,879.6
Unrealized losses, Total | (827.6) | (767.3)
U.S. Treasury bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 94.9 | 8.2
Unrealized losses, Less than 12 months | (1.6) | (0.1)
Estimated fair value, 12 months or longer | 55.7 | 55.4
Unrealized losses, 12 months or longer | (4.5) | (4.4)
Estimated fair value, Total | 150.6 | 63.6
Unrealized losses, Total | (6.1) | (4.5)
Municipal bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 99.3 | 107.4
Unrealized losses, Less than 12 months | (1.4) | (0.9)
Estimated fair value, 12 months or longer | 1,010.8 | 956.8
Unrealized losses, 12 months or longer | (141.8) | (135.8)
Estimated fair value, Total | 1,110.1 | 1,064.2
Unrealized losses, Total | (143.2) | (136.7)
Foreign government bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 57.3 | 33.3
Unrealized losses, Less than 12 months | (0.1) | (0.1)
Estimated fair value, 12 months or longer | 81.8 | 101.4
Unrealized losses, 12 months or longer | (10.0) | (10.4)
Estimated fair value, Total | 139.1 | 134.7
Unrealized losses, Total | (10.1) | (10.5)
Governmental agency bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 82.5 | 0.4
Unrealized losses, Less than 12 months | (0.7) | 0.0
Estimated fair value, 12 months or longer | 117.3 | 118.9
Unrealized losses, 12 months or longer | (13.2) | (12.5)
Estimated fair value, Total | 199.8 | 119.3
Unrealized losses, Total | (13.9) | (12.5)
Governmental agency mortgage-backed securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 363.7 | 338.3
Unrealized losses, Less than 12 months | (3.7) | (6.6)
Estimated fair value, 12 months or longer | 3,283.7 | 3,225.3
Unrealized losses, 12 months or longer | (571.7) | (520.2)
Estimated fair value, Total | 3,647.4 | 3,563.6
Unrealized losses, Total | (575.4) | (526.8)
U.S. corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 161.3 | 45.1
Unrealized losses, Less than 12 months | (1.9) | (0.4)
Estimated fair value, 12 months or longer | 587.9 | 602.5
Unrealized losses, 12 months or longer | (56.2) | (54.8)
Estimated fair value, Total | 749.2 | 647.6
Unrealized losses, Total | (58.1) | (55.2)
Foreign corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 68.5 | 19.3
Unrealized losses, Less than 12 months | (0.8) | (0.1)
Estimated fair value, 12 months or longer | 250.2 | 267.3
Unrealized losses, 12 months or longer | (20.0) | (21.0)
Estimated fair value, Total | 318.7 | 286.6
Unrealized losses, Total | $ (20.8) | $ (21.1)

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