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SUMMARY OF PRINCIPAL ACCOUNTING POLICIES - Allowance for doubtful receivables (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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Principal accounting policies: |  |  |
Prepaid expenses and other current assets | $ 353,781 | $ 253,542 |
Accounts Receivable, Allowance for Credit Loss | 9,874 | 1,250 | $ 2,812
Prepaid expenses and other current assets |  |  |
Principal accounting policies: |  |  |
Accounts Receivable, Allowance for Credit Loss | $ 386 | $ 166 |

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