FINANCIAL INSTRUMENTS - Schedule of Amortized Cost and Fair Value of Cash Equivalents, Investments, and Restricted Cash and Investments with Contractual Maturities (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Cost
Due in one year or less	$ 5,391,453
Due after one year through five years	380,219
Due in more than five years	47,571
Debt securities, Cost	5,819,243
Estimated Fair Value
Due in one year or less	5,391,323
Due after one year through five years	380,361
Due in more than five years	47,596
Total	$ 5,819,280