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Investments (Securities in a gross unrealized loss position by ratings) (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Estimated Fair Value | $ 31,117 | $ 29,511
Gross Unrealized Losses | 4,888 | 4,056
U.S. Government, Government agencies and Government-sponsored enterprises |  |
Debt Securities, Available-for-sale [Line Items] |  |
Estimated Fair Value | 2,620 | 2,355
Gross Unrealized Losses | 466 | 337
AAA |  |
Debt Securities, Available-for-sale [Line Items] |  |
Estimated Fair Value | 1,893 | 1,559
Gross Unrealized Losses | 382 | 298
AA |  |
Debt Securities, Available-for-sale [Line Items] |  |
Estimated Fair Value | 4,458 | 4,327
Gross Unrealized Losses | 1,037 | 817
A |  |
Debt Securities, Available-for-sale [Line Items] |  |
Estimated Fair Value | 7,240 | 6,615
Gross Unrealized Losses | 913 | 749
BBB |  |
Debt Securities, Available-for-sale [Line Items] |  |
Estimated Fair Value | 13,704 | 13,226
Gross Unrealized Losses | 1,881 | 1,621
Non-investment grade |  |
Debt Securities, Available-for-sale [Line Items] |  |
Estimated Fair Value | 1,202 | 1,429
Gross Unrealized Losses | $ 209 | $ 234

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