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INVESTMENTS - Summary of Investments by Major Security Type (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020 | Sep. 30, 2020
--- | --- | --- | ---
Long-term investments: |  |  |
Carrying Value | $ 3,283 | $ 3,759 |
Trip.com Group |  |  |
Equity security investments with readily determinable fair value: |  |  |
Cost |  |  | $ 655
Convertible Debt Securities | Trip.com Group |  |  |
Debt Securities, Available-for-sale [Abstract] |  |  |
Cost | 525 |  |
Short-term Investments | Convertible Debt Securities | Trip.com Group |  |  |
Debt Securities, Available-for-sale [Abstract] |  |  |
Cost | 525 | 500 |
Gross Unrealized Gains/Upward Adjustments | 0 | 1 |
Gross Unrealized Losses/Downward Adjustments | (3) | 0 |
Carrying Value | 522 | 501 |
Long-term investments |  |  |
Long-term investments: |  |  |
Cost | 1,229 | 1,240 |
Gross Unrealized Gains /Upward Adjustments | 2,248 | 2,620 |
Gross Unrealized Losses /Downward Adjustments | (194) | (101) |
Carrying Value | 3,283 | 3,759 |
Long-term investments | Convertible Debt Securities | Trip.com Group |  |  |
Debt Securities, Available-for-sale [Abstract] |  |  |
Cost |  | 25 |
Gross Unrealized Gains/Upward Adjustments |  | 0 |
Gross Unrealized Losses/Downward Adjustments |  | (1) |
Carrying Value |  | 24 |
Long-term investments | Investments in private company debt securities |  |  |
Debt Securities, Available-for-sale [Abstract] |  |  |
Cost | 200 | 200 |
Gross Unrealized Gains/Upward Adjustments | 130 | 0 |
Gross Unrealized Losses/Downward Adjustments | 0 | 0 |
Carrying Value | 330 | 200 |
Long-term investments | Investment in private company equity securities |  |  |
Equity security investments in private companies: |  |  |
Cost | 66 | 552 |
Gross Unrealized Gains /Upward Adjustments | 3 | 3 |
Gross Unrealized Losses /Downward Adjustments | 0 | (100) |
Carrying Value | 69 | 455 |
Long-term investments | Equity securities |  |  |
Equity security investments with readily determinable fair value: |  |  |
Cost | 963 | 463 |
Gross Unrealized Gains /Upward Adjustments | 2,115 | 2,617 |
Gross Unrealized Losses /Downward Adjustments | (194) | 0 |
Carrying Value | $ 2,884 | $ 3,080 |

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