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Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
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Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | $ 552.0 | $ 3,865.7
Unrealized losses, Less than 12 months | (8.2) | (268.4)
Estimated fair value, 12 months or longer | 5,327.6 | 3,780.1
Unrealized losses, 12 months or longer | (759.1) | (741.1)
Estimated fair value, Total | 5,879.6 | 7,645.8
Unrealized losses, Total | (767.3) | (1,009.5)
U.S. Treasury bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 8.2 | 108.0
Unrealized losses, Less than 12 months | (0.1) | (1.4)
Estimated fair value, 12 months or longer | 55.4 | 49.5
Unrealized losses, 12 months or longer | (4.4) | (5.7)
Estimated fair value, Total | 63.6 | 157.5
Unrealized losses, Total | (4.5) | (7.1)
Municipal bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 107.4 | 813.4
Unrealized losses, Less than 12 months | (0.9) | (55.8)
Estimated fair value, 12 months or longer | 956.8 | 540.0
Unrealized losses, 12 months or longer | (135.8) | (139.3)
Estimated fair value, Total | 1,064.2 | 1,353.4
Unrealized losses, Total | (136.7) | (195.1)
Foreign government bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 33.3 | 142.1
Unrealized losses, Less than 12 months | (0.1) | (5.9)
Estimated fair value, 12 months or longer | 101.4 | 45.7
Unrealized losses, 12 months or longer | (10.4) | (8.5)
Estimated fair value, Total | 134.7 | 187.8
Unrealized losses, Total | (10.5) | (14.4)
Governmental agency bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 0.4 | 172.7
Unrealized losses, Less than 12 months | 0.0 | (8.2)
Estimated fair value, 12 months or longer | 118.9 | 55.8
Unrealized losses, 12 months or longer | (12.5) | (11.1)
Estimated fair value, Total | 119.3 | 228.5
Unrealized losses, Total | (12.5) | (19.3)
Governmental agency mortgage-backed securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 338.3 | 1,859.6
Unrealized losses, Less than 12 months | (6.6) | (141.4)
Estimated fair value, 12 months or longer | 3,225.3 | 2,626.8
Unrealized losses, 12 months or longer | (520.2) | (511.3)
Estimated fair value, Total | 3,563.6 | 4,486.4
Unrealized losses, Total | (526.8) | (652.7)
U.S. corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 45.1 | 528.3
Unrealized losses, Less than 12 months | (0.4) | (38.2)
Estimated fair value, 12 months or longer | 602.5 | 325.2
Unrealized losses, 12 months or longer | (54.8) | (46.3)
Estimated fair value, Total | 647.6 | 853.5
Unrealized losses, Total | (55.2) | (84.5)
Foreign corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 19.3 | 241.6
Unrealized losses, Less than 12 months | (0.1) | (17.5)
Estimated fair value, 12 months or longer | 267.3 | 137.1
Unrealized losses, 12 months or longer | (21.0) | (18.9)
Estimated fair value, Total | 286.6 | 378.7
Unrealized losses, Total | $ (21.1) | $ (36.4)

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