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CASH, CASH EQUIVALENTS AND SHORT AND LONG TERM INVESTMENTS - Long-term Investments, UTStarcom Hong Kong Holdings Ltd - Additional Information (Details) - UTStarcom Hong Kong Holdings Ltd. - USD ($) $ in Millions |  |  |  |  | 12 Months Ended
--- | --- | --- | --- | --- | ---
Mar. 31, 2021 | Dec. 17, 2020 | Apr. 07, 2015 | Aug. 31, 2012 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2015
Convertible bonds of private company |  |  |  |  |  |  |  |
Payments to Acquire Investments |  |  |  | $ 20.0 |  |  |  |
Interest rate of debt securities (as a percent) |  |  |  | 6.50% |  |  |  |
Face amount of convertible debt |  |  | $ 20.0 |  |  |  |  |
Proceeds from sale of bond |  |  | $ 10.0 |  |  |  |  |
Ownership interest upon conversion of preference shares and convertible bonds |  |  | 14.00% |  |  |  |  |
Impairment charge on investment |  |  |  |  |  |  | $ 0.4 | $ 6.5
Carrying amount of the investment |  |  |  |  |  | $ 3.1 |  |
Eagle Field Holding Limited |  |  |  |  |  |  |  |
Impairment charge on investment |  |  |  |  | $ 1.0 |  |  |
Equity interest percentage |  | 14.00% |  |  |  |  |  |
Aggregate cash consideration | $ 2.1 |  |  |  |  |  |  |
Amount received from buyer | $ 2.1 |  |  |  |  |  |  |

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