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Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions | Jun. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | $ 1,728.7 | $ 3,865.7
Unrealized losses, Less than 12 months | (57.7) | (268.4)
Estimated fair value, 12 months or longer | 5,261.6 | 3,780.1
Unrealized losses, 12 months or longer | (911.2) | (741.1)
Estimated fair value, Total | 6,990.3 | 7,645.8
Unrealized losses, Total | (968.9) | (1,009.5)
U.S. Treasury bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 58.9 | 108.0
Unrealized losses, Less than 12 months | (0.6) | (1.4)
Estimated fair value, 12 months or longer | 54.7 | 49.5
Unrealized losses, 12 months or longer | (5.9) | (5.7)
Estimated fair value, Total | 113.6 | 157.5
Unrealized losses, Total | (6.5) | (7.1)
Municipal bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 315.3 | 813.4
Unrealized losses, Less than 12 months | (6.6) | (55.8)
Estimated fair value, 12 months or longer | 822.1 | 540.0
Unrealized losses, 12 months or longer | (157.0) | (139.3)
Estimated fair value, Total | 1,137.4 | 1,353.4
Unrealized losses, Total | (163.6) | (195.1)
Foreign government bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 88.6 | 142.1
Unrealized losses, Less than 12 months | (1.0) | (5.9)
Estimated fair value, 12 months or longer | 88.0 | 45.7
Unrealized losses, 12 months or longer | (12.9) | (8.5)
Estimated fair value, Total | 176.6 | 187.8
Unrealized losses, Total | (13.9) | (14.4)
Governmental agency bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 113.7 | 172.7
Unrealized losses, Less than 12 months | (2.9) | (8.2)
Estimated fair value, 12 months or longer | 110.7 | 55.8
Unrealized losses, 12 months or longer | (14.9) | (11.1)
Estimated fair value, Total | 224.4 | 228.5
Unrealized losses, Total | (17.8) | (19.3)
Governmental agency mortgage-backed securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 884.3 | 1,859.6
Unrealized losses, Less than 12 months | (40.7) | (141.4)
Estimated fair value, 12 months or longer | 3,304.5 | 2,626.8
Unrealized losses, 12 months or longer | (617.4) | (511.3)
Estimated fair value, Total | 4,188.8 | 4,486.4
Unrealized losses, Total | (658.1) | (652.7)
U.S. corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 174.3 | 528.3
Unrealized losses, Less than 12 months | (3.6) | (38.2)
Estimated fair value, 12 months or longer | 604.1 | 325.2
Unrealized losses, 12 months or longer | (73.3) | (46.3)
Estimated fair value, Total | 778.4 | 853.5
Unrealized losses, Total | (76.9) | (84.5)
Foreign corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 93.6 | 241.6
Unrealized losses, Less than 12 months | (2.3) | (17.5)
Estimated fair value, 12 months or longer | 277.5 | 137.1
Unrealized losses, 12 months or longer | (29.8) | (18.9)
Estimated fair value, Total | 371.1 | 378.7
Unrealized losses, Total | $ (32.1) | $ (36.4)

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