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Cash and Cash Equivalents, Short-term Investments and Restricted Cash (Tables)
 6 Months Ended
Jun. 30, 2021
Cash and Cash Equivalents [Abstract]  
Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Market Value of Cash and Cash Equivalents and Available-for-Sale Investments The following tables present the amortized cost, gross unrealized gains and losses and estimated fair market value of our cash and cash equivalents, short-term investments and restricted cash as of the dates presented (in thousands):
 June 30, 2021
 Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Market
Value
Cash$218,514 $— $— $218,514 
Cash equivalents:
Money market funds3,494,642 — — 3,494,642 
Short-term investments:
U. S. government agency securities829,791 134 — 829,925 
Treasury bills44,975 13 — 44,988 
Municipal securities5,962 — — 5,962 
Restricted cash141,024 — — 141,024 
        Total$4,734,908 $147 $— $4,735,055 
 December 31, 2020
 Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair Market
Value
Cash$213,518 $— $— $213,518 
Cash equivalents:
Money market funds1,486,384 — — 1,486,384 
Municipal securities3,229 — (1)3,228 
Short-term investments:
Treasury bills1,163,748 65 — 1,163,813 
U.S. government agency securities1,037,572 57 (52)1,037,577 
Municipal securities16,226 — (6)16,220 
Certificates of deposit498 — — 498 
Restricted cash75,805 — — 75,805 
Total$3,996,980 $122 $(59)$3,997,043