FAIR VALUE MEASUREMENTS (Reported Amounts and Estimated Fair Values of the Financial Assets and Liabilities for Which Disclosure of Fair Value is Required) (Details) - Nonrecurring - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Balance
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	$ 417,819	$ 282,664
Restricted cash	1,136	1,151
Employee loans	2,434	3,525
Balance | Revolving Credit Facility | 2017 Credit Facility
Liabilities, Fair Value Disclosure [Abstract]
Borrowings	25,017	25,020
Balance | Money Market Funds
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	407,817	282,664
Balance | Time Deposits
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	10,002
Estimated Fair Value
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	417,819	282,664
Restricted cash	1,136	1,151
Employee loans	2,434	3,525
Estimated Fair Value | Revolving Credit Facility | 2017 Credit Facility
Liabilities, Fair Value Disclosure [Abstract]
Borrowings	25,017	25,020
Estimated Fair Value | Money Market Funds
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	407,817	282,664
Estimated Fair Value | Time Deposits
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	10,002
Estimated Fair Value | Level 1
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	407,817	282,664
Restricted cash	1,136	1,151
Employee loans	0	0
Estimated Fair Value | Level 1 | Revolving Credit Facility | 2017 Credit Facility
Liabilities, Fair Value Disclosure [Abstract]
Borrowings	0	0
Estimated Fair Value | Level 1 | Money Market Funds
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	407,817	282,664
Estimated Fair Value | Level 1 | Time Deposits
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0
Estimated Fair Value | Level 2
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	10,002	0
Restricted cash	0	0
Employee loans	0	0
Estimated Fair Value | Level 2 | Revolving Credit Facility | 2017 Credit Facility
Liabilities, Fair Value Disclosure [Abstract]
Borrowings	25,017	25,020
Estimated Fair Value | Level 2 | Money Market Funds
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0	0
Estimated Fair Value | Level 2 | Time Deposits
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	10,002
Estimated Fair Value | Level 3
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0	0
Restricted cash	0	0
Employee loans	2,434	3,525
Estimated Fair Value | Level 3 | Revolving Credit Facility | 2017 Credit Facility
Liabilities, Fair Value Disclosure [Abstract]
Borrowings	0	0
Estimated Fair Value | Level 3 | Money Market Funds
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	0	$ 0
Estimated Fair Value | Level 3 | Time Deposits
Assets, Fair Value Disclosure [Abstract]
Cash equivalents	$ 0