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Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions | Mar. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | $ 1,763.2 | $ 3,865.7
Unrealized losses, Less than 12 months | (69.9) | (268.4)
Estimated fair value, 12 months or longer | 5,112.5 | 3,780.1
Unrealized losses, 12 months or longer | (799.9) | (741.1)
Estimated fair value, Total | 6,875.7 | 7,645.8
Unrealized losses, Total | (869.8) | (1,009.5)
U.S. Treasury bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 40.1 | 108.0
Unrealized losses, Less than 12 months | (0.1) | (1.4)
Estimated fair value, 12 months or longer | 55.5 | 49.5
Unrealized losses, 12 months or longer | (5.3) | (5.7)
Estimated fair value, Total | 95.6 | 157.5
Unrealized losses, Total | (5.4) | (7.1)
Municipal bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 299.8 | 813.4
Unrealized losses, Less than 12 months | (5.9) | (55.8)
Estimated fair value, 12 months or longer | 794.6 | 540.0
Unrealized losses, 12 months or longer | (147.9) | (139.3)
Estimated fair value, Total | 1,094.4 | 1,353.4
Unrealized losses, Total | (153.8) | (195.1)
Foreign government bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 45.3 | 142.1
Unrealized losses, Less than 12 months | (0.4) | (5.9)
Estimated fair value, 12 months or longer | 96.8 | 45.7
Unrealized losses, 12 months or longer | (10.8) | (8.5)
Estimated fair value, Total | 142.1 | 187.8
Unrealized losses, Total | (11.2) | (14.4)
Governmental agency bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 146.2 | 172.7
Unrealized losses, Less than 12 months | (3.3) | (8.2)
Estimated fair value, 12 months or longer | 84.1 | 55.8
Unrealized losses, 12 months or longer | (13.0) | (11.1)
Estimated fair value, Total | 230.3 | 228.5
Unrealized losses, Total | (16.3) | (19.3)
Governmental agency mortgage-backed securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 1,004.1 | 1,859.6
Unrealized losses, Less than 12 months | (53.3) | (141.4)
Estimated fair value, 12 months or longer | 3,230.9 | 2,626.8
Unrealized losses, 12 months or longer | (528.6) | (511.3)
Estimated fair value, Total | 4,235.0 | 4,486.4
Unrealized losses, Total | (581.9) | (652.7)
U.S. corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 142.4 | 528.3
Unrealized losses, Less than 12 months | (4.3) | (38.2)
Estimated fair value, 12 months or longer | 582.4 | 325.2
Unrealized losses, 12 months or longer | (66.7) | (46.3)
Estimated fair value, Total | 724.8 | 853.5
Unrealized losses, Total | (71.0) | (84.5)
Foreign corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Estimated fair value, Less than 12 months | 85.3 | 241.6
Unrealized losses, Less than 12 months | (2.6) | (17.5)
Estimated fair value, 12 months or longer | 268.2 | 137.1
Unrealized losses, 12 months or longer | (27.6) | (18.9)
Estimated fair value, Total | 353.5 | 378.7
Unrealized losses, Total | $ (30.2) | $ (36.4)

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