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Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
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Investments, Unrealized Loss Position |  |
Investments, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months |  | $ 6,683
Investments, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 121
Investments, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 5,823
Investments, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 510
Investments, Available-for-Sale, Unrealized Loss Position |  | 12,506
Investments, Available-for-Sale, Unrealized Loss Position, Accumulated Loss |  | 631
Short-term investments, available-for-sale, continuous unrealized loss position, less than 12 months |  | 100
Short-term Investments, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 0
Short-term investments, available-for-sale, continuous unrealized loss position, 12 months or longer |  | 0
Short-term investments, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss |  | 0
Short-term investments, available-for-sale, unrealized loss position, total |  | 100
Short-term investments, available-for-sale, unrealized loss position, accumulated loss |  | 0
Less than 12 months Fair Value | $ 2,762 | 6,583
Less than 12 Months, Unrealized Loss | 41 | 121
12 months or more, Fair Value | 5,512 | 5,823
12 months or longer, Unrealized Loss | 388 | 510
Total Fair Value | 8,274 | 12,406
Total Unrealized Losses | 429 | 631
Corporate |  |
Investments, Unrealized Loss Position |  |
Less than 12 months Fair Value | 793 | 2,815
Less than 12 Months, Unrealized Loss | 14 | 78
12 months or more, Fair Value | 2,887 | 3,634
12 months or longer, Unrealized Loss | 179 | 255
Total Fair Value | 3,680 | 6,449
Total Unrealized Losses | 193 | 333
States, municipalities and political subdivisions |  |
Investments, Unrealized Loss Position |  |
Less than 12 months Fair Value | 873 | 1,513
Less than 12 Months, Unrealized Loss | 20 | 25
12 months or more, Fair Value | 2,019 | 1,898
12 months or longer, Unrealized Loss | 201 | 245
Total Fair Value | 2,892 | 3,411
Total Unrealized Losses | 221 | 270
Government-sponsored enterprises |  |
Investments, Unrealized Loss Position |  |
Less than 12 months Fair Value | 922 | 1,876
Less than 12 Months, Unrealized Loss | 3 | 8
12 months or more, Fair Value | 488 | 92
12 months or longer, Unrealized Loss | 2 | 1
Total Fair Value | 1,410 | 1,968
Total Unrealized Losses | 5 | 9
Asset-backed |  |
Investments, Unrealized Loss Position |  |
Less than 12 months Fair Value | 173 | 331
Less than 12 Months, Unrealized Loss | 4 | 10
12 months or more, Fair Value | 91 | 96
12 months or longer, Unrealized Loss | 5 | 7
Total Fair Value | 264 | 427
Total Unrealized Losses | 9 | 17
United States government |  |
Investments, Unrealized Loss Position |  |
Less than 12 months Fair Value | 0 | 48
Less than 12 Months, Unrealized Loss | 0 | 0
12 months or more, Fair Value | 27 | 100
12 months or longer, Unrealized Loss | 1 | 2
Total Fair Value | 27 | 148
Total Unrealized Losses | 1 | 2
Foreign government |  |
Investments, Unrealized Loss Position |  |
Less than 12 months Fair Value | 1 | 0
Less than 12 Months, Unrealized Loss | 0 | 0
12 months or more, Fair Value | 0 | 3
12 months or longer, Unrealized Loss | 0 | 0
Total Fair Value | 1 | 3
Total Unrealized Losses | $ 0 | $ 0

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