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Short-term and Long-term Investments, Restricted Deposits - Narrative (Details) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2021 USD ($) position | Jul. 31, 2021 USD ($) | Sep. 30, 2021 USD ($) position | Sep. 30, 2021 USD ($) position | Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($) | Jun. 30, 2019
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |  |  |  |  |
Accrued interest income | $ 92 |  | $ 92 | $ 92 |  | $ 86 |
Positions from which gross unrealized losses were generated | position | 1,887 |  | 1,887 | 1,887 |  |  |
Total unrealized investment positions | position | 6,468 |  | 6,468 | 6,468 |  |  |
Step acquisition, gain on investment |  |  |  | $ 309 | $ 0 |  |
Circle Health |  |  |  |  |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |  |  |  |  |
Acquisition, interests acquired |  | 60.00% |  |  |  |  |
Business combination, ownership percentage |  | 100.00% |  |  |  |  |
Circle Health |  |  |  |  |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |  |  |  |  |
Equity method investment, ownership percentage |  |  |  |  |  |  | 40.00%
Cash paid |  | $ 705 |  |  |  |  |
Step acquisition, gain on investment |  |  | $ 309 |  |  |  |
RxAdvance |  |  |  |  |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |  |  |  |  |
Impairment of equity method investment | $ 229 |  |  |  |  |  |
Commercial mortgage-backed securities |  |  |  |  |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |  |  |  |  |
Investments recorded at fair value that carry rating of AA Plus, weighted average (in years) |  |  |  | 4 years |  |  |
Rated Securities | External Credit Rating, Investment Grade | Investment Securities |  |  |  |  |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |  |  |  |  |
Percentage of investments in rated securities carry an investment grade rating by nationally recognized statistical rating organizations |  |  |  | 98.00% |  |  |

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