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Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
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Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | $ 1,067.8 | $ 552.0
Unrealized losses, Less than 12 months | (21.2) | (8.2)
Estimated fair value, 12 months or longer | 5,194.2 | 5,327.6
Unrealized losses, 12 months or longer | (801.3) | (759.1)
Estimated fair value, Total | 6,262.0 | 5,879.6
Unrealized losses, Total | (822.5) | (767.3)
U.S. Treasury bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 101.2 | 8.2
Unrealized losses, Less than 12 months | (1.3) | (0.1)
Estimated fair value, 12 months or longer | 50.4 | 55.4
Unrealized losses, 12 months or longer | (4.6) | (4.4)
Estimated fair value, Total | 151.6 | 63.6
Unrealized losses, Total | (5.9) | (4.5)
Municipal bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 133.1 | 107.4
Unrealized losses, Less than 12 months | (1.5) | (0.9)
Estimated fair value, 12 months or longer | 970.8 | 956.8
Unrealized losses, 12 months or longer | (137.5) | (135.8)
Estimated fair value, Total | 1,103.9 | 1,064.2
Unrealized losses, Total | (139.0) | (136.7)
Foreign government bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 68.3 | 33.3
Unrealized losses, Less than 12 months | (0.2) | (0.1)
Estimated fair value, 12 months or longer | 96.1 | 101.4
Unrealized losses, 12 months or longer | (10.5) | (10.4)
Estimated fair value, Total | 164.4 | 134.7
Unrealized losses, Total | (10.7) | (10.5)
Governmental agency bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 83.0 | 0.4
Unrealized losses, Less than 12 months | (0.6) | 0.0
Estimated fair value, 12 months or longer | 117.4 | 118.9
Unrealized losses, 12 months or longer | (13.3) | (12.5)
Estimated fair value, Total | 200.4 | 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 | 490.3 | 338.3
Unrealized losses, Less than 12 months | (15.4) | (6.6)
Estimated fair value, 12 months or longer | 3,103.7 | 3,225.3
Unrealized losses, 12 months or longer | (557.6) | (520.2)
Estimated fair value, Total | 3,594.0 | 3,563.6
Unrealized losses, Total | (573.0) | (526.8)
U.S. corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 127.1 | 45.1
Unrealized losses, Less than 12 months | (1.6) | (0.4)
Estimated fair value, 12 months or longer | 598.1 | 602.5
Unrealized losses, 12 months or longer | (57.0) | (54.8)
Estimated fair value, Total | 725.2 | 647.6
Unrealized losses, Total | (58.6) | (55.2)
Foreign corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 64.8 | 19.3
Unrealized losses, Less than 12 months | (0.6) | (0.1)
Estimated fair value, 12 months or longer | 257.7 | 267.3
Unrealized losses, 12 months or longer | (20.8) | (21.0)
Estimated fair value, Total | 322.5 | 286.6
Unrealized losses, Total | $ (21.4) | $ (21.1)

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