Financial Instruments - Cost, Amortized Cost, Gross Unrealized Gains & Losses, and Estimated Fair Market Value (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cost or Amortized Cost
Cash and cash equivalents, at carrying value	$ 925,276	$ 234,679
Restricted cash		12,769
Cash, cash equivalents, and available-for-sale debt securities, amortized cost		348,446
Unrealized Gains (Losses)
Unrealized Gains		31
Unrealized Losses		(22)
Estimated Fair Value
Restricted cash		12,769
Cash, cash equivalents, and available-for-sale debt securities		348,455
Cash, Cash Equivalents, Restricted Cash
Cash, Cash Equivalents, Restricted Cash	945,820	247,448	$ 439,055	$ 212,658
Short-term Investments
Short-term Investments		70,029
Long-term Investments
Long-term Investments		30,978
Fair Value, Measurements, Recurring
Cost or Amortized Cost
Restricted cash	20,544	12,769
Cash, cash equivalents, and available-for-sale debt securities, amortized cost	1,089,122
Unrealized Gains (Losses)
Unrealized Gains	181
Unrealized Losses	0
Estimated Fair Value
Cash and cash equivalents, fair value disclosure	892,361
Restricted cash	20,544	12,769
Cash, cash equivalents, and available-for-sale debt securities	1,089,303
Cash, Cash Equivalents, Restricted Cash
Cash, Cash Equivalents, Restricted Cash	945,820	247,448
Short-term Investments
Short-term Investments	131,561
Long-term Investments
Long-term Investments	11,922
U.S. treasury securities | Fair Value, Measurements, Recurring
Cost or Amortized Cost
Debt securities, available-for-sale, amortized cost	137,502	100,998
Unrealized Gains (Losses)
Unrealized Gains	159	31
Unrealized Losses	0	(22)
Estimated Fair Value
Debt securities, available-for-sale	137,661	101,007
Short-term Investments
Short-term Investments	131,561	70,029
Long-term Investments
Long-term Investments		30,978
Agency bonds | Fair Value, Measurements, Recurring
Cost or Amortized Cost
Debt securities, available-for-sale, amortized cost	11,900
Unrealized Gains (Losses)
Unrealized Gains	22
Unrealized Losses	0
Estimated Fair Value
Debt securities, available-for-sale	11,922
Long-term Investments
Long-term Investments	11,922
Cash | Fair Value, Measurements, Recurring
Cost or Amortized Cost
Cash and cash equivalents, at carrying value	32,915	13,237
Estimated Fair Value
Cash and cash equivalents, fair value disclosure	32,915	13,237
Cash, Cash Equivalents, Restricted Cash
Cash, Cash Equivalents, Restricted Cash	32,915	13,237
Money markets funds | Fair Value, Measurements, Recurring
Cost or Amortized Cost
Cash and cash equivalents, at carrying value	886,261	221,442
Estimated Fair Value
Cash and cash equivalents, fair value disclosure	886,261	221,442
Cash, Cash Equivalents, Restricted Cash
Cash, Cash Equivalents, Restricted Cash	886,261	$ 221,442
U.S. treasury securities | Fair Value, Measurements, Recurring
Estimated Fair Value
Cash and cash equivalents, fair value disclosure	6,100
Cash, Cash Equivalents, Restricted Cash
Cash, Cash Equivalents, Restricted Cash	$ 6,100