Financial Instruments - Schedule of Amortized Cost and Fair Value of Cash Equivalents, Short-Term Investments, and Restricted Cash and Investments with Contractual Maturities (Details) $ in Thousands	Jun. 30, 2019 USD ($)
Cost
Due in one year or less	$ 4,842,996
Due after one year through five years	331,707
Due in more than five years	41,612
Debt securities, Cost	5,216,315
Estimated Fair Value
Due in one year or less	4,844,145
Due after one year through five years	333,019
Due in more than five years	41,756
Estimated fair value due, Total	$ 5,218,920