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FAIR VALUE MEASUREMENTS - Rollforward of Level 3 Fair Value Measurements (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Jun. 30, 2020 | Mar. 31, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | [1]
Debt Securities |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |
Balance, beginning of year |  |  | $ 250 | [1] | $ 200 | $ 250
Unrealized gains included in accumulated other comprehensive loss | [2] |  |  |  | 130 | 0
Balance, end of period |  |  |  |  | $ 330 | $ 250
Grab | Redeemable Convertible Preferred Stock | Long-term Investments |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |  |
Unrealized gain (loss) on debt securities |  | $ 20 | $ (20) |  |  |
[1] | The Company recognized an unrealized loss of $20 million during the three months ended March 31, 2020 and an unrealized gain of $20 million during the three months ended June 30, 2020 related to the investment in Grab.
[2] | The unrealized gains are recorded in "Accumulated other comprehensive loss" in the Consolidated Balance Sheet.

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