FAIR VALUE MEASUREMENTS (Financial Assets and Liabilities Not Measured on Recurring Basis) (Details) - Nonrecurring - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Balance
Financial Assets:
Cash and cash equivalents	$ 584,081	$ 582,585
Restricted cash	1,195	673
Employee loans	3,753	4,210
Balance | Revolving Credit Facility | Credit Facility 2017
Financial Liabilities:
Borrowings	25,019	25,009
Estimated Fair Value
Financial Assets:
Cash and cash equivalents	584,081	582,585
Restricted cash	1,195	673
Employee loans	3,753	4,210
Estimated Fair Value | Revolving Credit Facility | Credit Facility 2017
Financial Liabilities:
Borrowings	25,019	25,009
Estimated Fair Value | Level 1
Financial Assets:
Cash and cash equivalents	584,081	582,585
Restricted cash	1,195	0
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 and cash equivalents	0	0
Restricted cash	0	673
Employee loans	0	0
Estimated Fair Value | Level 2 | Revolving Credit Facility | Credit Facility 2017
Financial Liabilities:
Borrowings	25,019	25,009
Estimated Fair Value | Level 3
Financial Assets:
Cash and cash equivalents	0	0
Restricted cash	0	0
Employee loans	3,753	4,210
Estimated Fair Value | Level 3 | Revolving Credit Facility | Credit Facility 2017
Financial Liabilities:
Borrowings	$ 0	$ 0