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Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions | Jun. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | $ 3,017.3 | $ 4,076.1
Unrealized losses, Less than 12 months | (60.6) | (122.9)
Estimated fair value, 12 months or longer | 1,972.6 | 2,098.9
Unrealized losses, 12 months or longer | (300.9) | (361.7)
Estimated fair value, Total | 4,989.9 | 6,175.0
Unrealized losses, Total | (361.5) | (484.6)
U.S. Treasury bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 9.4 | 81.8
Unrealized losses, Less than 12 months | (0.2) | (1.8)
Estimated fair value, 12 months or longer | 41.8 | 52.9
Unrealized losses, 12 months or longer | (1.9) | (3.4)
Estimated fair value, Total | 51.2 | 134.7
Unrealized losses, Total | (2.1) | (5.2)
Municipal bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 228.1 | 248.3
Unrealized losses, Less than 12 months | (9.5) | (4.4)
Estimated fair value, 12 months or longer | 391.6 | 417.0
Unrealized losses, 12 months or longer | (65.0) | (69.3)
Estimated fair value, Total | 619.7 | 665.3
Unrealized losses, Total | (74.5) | (73.7)
Foreign government bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 29.7 | 29.8
Unrealized losses, Less than 12 months | 0.0 | (0.2)
Estimated fair value, 12 months or longer | 62.1 | 72.7
Unrealized losses, 12 months or longer | (5.8) | (6.9)
Estimated fair value, Total | 91.8 | 102.5
Unrealized losses, Total | (5.8) | (7.1)
Governmental agency bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 104.1 | 144.4
Unrealized losses, Less than 12 months | (0.4) | (5.3)
Estimated fair value, 12 months or longer | 37.2 | 37.9
Unrealized losses, 12 months or longer | (7.6) | (8.7)
Estimated fair value, Total | 141.3 | 182.3
Unrealized losses, Total | (8.0) | (14.0)
Governmental agency mortgage-backed securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 2,290.7 | 2,977.2
Unrealized losses, Less than 12 months | (47.0) | (98.4)
Estimated fair value, 12 months or longer | 1,246.4 | 1,290.4
Unrealized losses, 12 months or longer | (198.4) | (245.4)
Estimated fair value, Total | 3,537.1 | 4,267.6
Unrealized losses, Total | (245.4) | (343.8)
U.S. corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 258.6 | 435.2
Unrealized losses, Less than 12 months | (2.9) | (9.6)
Estimated fair value, 12 months or longer | 105.6 | 117.5
Unrealized losses, 12 months or longer | (15.6) | (18.8)
Estimated fair value, Total | 364.2 | 552.7
Unrealized losses, Total | (18.5) | (28.4)
Foreign corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 96.7 | 159.4
Unrealized losses, Less than 12 months | (0.6) | (3.2)
Estimated fair value, 12 months or longer | 87.9 | 110.5
Unrealized losses, 12 months or longer | (6.6) | (9.2)
Estimated fair value, Total | 184.6 | 269.9
Unrealized losses, Total | $ (7.2) | $ (12.4)

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