FAIR VALUE MEASUREMENTS (Estimated Fair Values of the Financial Assets and Liabilities not Measured at Fair Value on a Recurring Basis) (Details) - Nonrecurring - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Level 1
Financial Assets:
Cash equivalents:		$ 407,817
Restricted cash		1,136
Employee loans		0
Level 1 | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings		0
Level 2
Financial Assets:
Cash equivalents:		10,002
Restricted cash		0
Employee loans		0
Level 2 | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings		25,017
Level 3
Financial Assets:
Cash equivalents:		0
Restricted cash		0
Employee loans		2,434
Level 3 | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings		0
Balance
Financial Assets:
Cash equivalents:	$ 189,873	417,819
Restricted cash	1,157	1,136
Employee loans	1,491	2,434
Balance | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings	25,007	25,017
Estimated Fair Value
Financial Assets:
Cash equivalents:	189,873	417,819
Restricted cash	1,157	1,136
Employee loans	1,491	2,434
Estimated Fair Value | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings	25,007	25,017
Estimated Fair Value | Level 1
Financial Assets:
Cash equivalents:	189,873
Restricted cash	1,157	1,136
Employee loans	0	0
Estimated Fair Value | Level 1 | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings	0	0
Estimated Fair Value | Level 2
Financial Assets:
Cash equivalents:	0
Restricted cash	0	0
Employee loans	0	0
Estimated Fair Value | Level 2 | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings	25,007	25,017
Estimated Fair Value | Level 3
Financial Assets:
Cash equivalents:	0
Restricted cash	0	0
Employee loans	1,491	2,434
Estimated Fair Value | Level 3 | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings	0	0
Money market funds | Level 1
Financial Assets:
Cash equivalents:		407,817
Money market funds | Level 2
Financial Assets:
Cash equivalents:		0
Money market funds | Level 3
Financial Assets:
Cash equivalents:		0
Money market funds | Balance
Financial Assets:
Cash equivalents:	189,873	407,817
Money market funds | Estimated Fair Value
Financial Assets:
Cash equivalents:	189,873	407,817
Money market funds | Estimated Fair Value | Level 1
Financial Assets:
Cash equivalents:	189,873	407,817
Money market funds | Estimated Fair Value | Level 2
Financial Assets:
Cash equivalents:	0	0
Money market funds | Estimated Fair Value | Level 3
Financial Assets:
Cash equivalents:	0	0
Time deposits | Level 1
Financial Assets:
Cash equivalents:		0
Time deposits | Level 2
Financial Assets:
Cash equivalents:		10,002
Time deposits | Level 3
Financial Assets:
Cash equivalents:		0
Time deposits | Balance
Financial Assets:
Cash equivalents:		10,002
Short-term investments	60,025
Time deposits | Estimated Fair Value
Financial Assets:
Cash equivalents:		10,002
Short-term investments	60,025
Time deposits | Estimated Fair Value | Level 1
Financial Assets:
Cash equivalents:		0
Short-term investments	0
Time deposits | Estimated Fair Value | Level 2
Financial Assets:
Cash equivalents:		10,002
Short-term investments	60,025
Time deposits | Estimated Fair Value | Level 3
Financial Assets:
Cash equivalents:		$ 0
Short-term investments	$ 0