FAIR VALUE MEASUREMENTS (Reported Amounts and Estimated Fair Values of the Financial Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Balance
Financial Assets:
Cash and cash equivalents	$ 853,241	$ 770,560
Restricted cash	1,148	1,151
Employee loans	2,608	3,525
Balance | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings	25,017	25,020
Estimated Fair Value
Financial Assets:
Cash and cash equivalents	853,241	770,560
Restricted cash	1,148	1,151
Employee loans	2,608	3,525
Estimated Fair Value | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings	25,017	25,020
Estimated Fair Value | Level 1
Financial Assets:
Cash and cash equivalents	853,241	770,560
Restricted cash	1,148	1,151
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	0
Employee loans	0	0
Estimated Fair Value | Level 2 | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings	25,017	25,020
Estimated Fair Value | Level 3
Financial Assets:
Cash and cash equivalents	0	0
Restricted cash	0	0
Employee loans	2,608	3,525
Estimated Fair Value | Level 3 | Revolving credit facility | Credit Facility 2017
Financial Liabilities:
Borrowings	$ 0	$ 0