FAIR VALUE MEASUREMENTS (Estimated Fair Values of the Financial Assets and Liabilities not Measured at Fair Value on a Recurring Basis) (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Balance
Financial Assets:
Cash equivalents:	$ 97,798	$ 153,783
Restricted cash	158,950	1,390
Employee loans	640	794
Balance | Money market funds
Financial Assets:
Cash equivalents:	97,798	153,783
Balance | Time deposits included in Short-term investments
Financial Assets:
Time deposits included in Short-term investments		60,007
Balance | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings under the 2017 Credit Facility	25,007	25,007
Estimated Fair Value
Financial Assets:
Cash equivalents:	97,798	153,783
Restricted cash	158,950	1,390
Employee loans	640	794
Estimated Fair Value | Money market funds
Financial Assets:
Cash equivalents:	97,798	153,783
Estimated Fair Value | Time deposits included in Short-term investments
Financial Assets:
Time deposits included in Short-term investments		60,007
Estimated Fair Value | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings under the 2017 Credit Facility	25,007	25,007
Estimated Fair Value | Level 1
Financial Assets:
Cash equivalents:	97,798	153,783
Restricted cash	158,950	1,390
Employee loans	0	0
Estimated Fair Value | Level 1 | Money market funds
Financial Assets:
Cash equivalents:	97,798	153,783
Estimated Fair Value | Level 1 | Time deposits included in Short-term investments
Financial Assets:
Time deposits included in Short-term investments		0
Estimated Fair Value | Level 1 | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings under the 2017 Credit Facility	0	0
Estimated Fair Value | Level 2
Financial Assets:
Cash equivalents:	0	0
Restricted cash	0	0
Employee loans	0	0
Estimated Fair Value | Level 2 | Money market funds
Financial Assets:
Cash equivalents:	0	0
Estimated Fair Value | Level 2 | Time deposits included in Short-term investments
Financial Assets:
Time deposits included in Short-term investments		60,007
Estimated Fair Value | Level 2 | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings under the 2017 Credit Facility	25,007	25,007
Estimated Fair Value | Level 3
Financial Assets:
Cash equivalents:	0	0
Restricted cash	0	0
Employee loans	640	794
Estimated Fair Value | Level 3 | Money market funds
Financial Assets:
Cash equivalents:	0	0
Estimated Fair Value | Level 3 | Time deposits included in Short-term investments
Financial Assets:
Time deposits included in Short-term investments		0
Estimated Fair Value | Level 3 | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings under the 2017 Credit Facility	$ 0	$ 0