Financial Instruments - Investments in Debt and Marketable Equity Securities Disclosure (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Cost or Amortized Cost
Cash and cash equivalents, at carrying value	$ 325,352	$ 234,679
Restricted cash	35,102	12,769
Cash, cash equivalents, and available-for-sale debt securities, amortized cost	506,872	348,446
Unrealized Gains (Losses)
Unrealized Gains	430	31
Unrealized Losses	0	(22)
Estimated Fair Value
Cash, cash equivalents, and available-for-sale debt securities	507,302	348,455
Short-term Investments
Short-term investments	128,056	70,029
Long-term Investments
Long-term investments	18,792	30,978
Cash, cash equivalents, restricted cash and restricted cash equivalents	360,454	247,448	$ 259,237	$ 439,055
Fair Value, Measurements, Recurring
Long-term Investments
Long-term investments	18,792
Money market funds	314,415
U.S. treasury securities | Fair Value, Measurements, Recurring
Short-term Investments
Short-term investments	128,056	70,029
Long-term Investments
Long-term investments	13,783	30,978
Agency bonds | Fair Value, Measurements, Recurring
Long-term Investments
Long-term investments	5,009
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Long-term Investments
Long-term investments	18,792
Money market funds	314,415
Fair Value, Inputs, Level 1 | U.S. treasury securities
Cost or Amortized Cost
Debt securities, available-for-sale, amortized cost	145,718	100,998
Unrealized Gains (Losses)
Unrealized Gains	421	31
Unrealized Losses	0	(22)
Estimated Fair Value
Debt securities, available-for-sale	146,139	101,007
Short-term Investments
Short-term investments	128,056	70,029
Long-term Investments
Long-term investments	13,783	30,978
Fair Value, Inputs, Level 1 | U.S. treasury securities | Fair Value, Measurements, Recurring
Short-term Investments
Short-term investments	128,056	70,029
Long-term Investments
Long-term investments	13,783	30,978
Fair Value, Inputs, Level 1 | Agency bonds
Cost or Amortized Cost
Debt securities, available-for-sale, amortized cost	5,000
Unrealized Gains (Losses)
Unrealized Gains	9
Unrealized Losses	0
Estimated Fair Value
Debt securities, available-for-sale	5,009
Short-term Investments
Short-term investments	0
Long-term Investments
Long-term investments	5,009
Fair Value, Inputs, Level 1 | Agency bonds | Fair Value, Measurements, Recurring
Long-term Investments
Long-term investments	5,009
Cash
Cost or Amortized Cost
Cash and cash equivalents, at carrying value	10,937	13,237
Money Market Funds | Fair Value, Measurements, Recurring
Long-term Investments
Money market funds	310,115	221,442
Money Market Funds | Fair Value, Inputs, Level 1
Cost or Amortized Cost
Cash and cash equivalents, at carrying value	310,115	221,442
Money Market Funds | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Long-term Investments
Money market funds	310,115	$ 221,442
U.S. treasury securities | Fair Value, Measurements, Recurring
Long-term Investments
Money market funds	4,300
U.S. treasury securities | Fair Value, Inputs, Level 1
Long-term Investments
Money market funds	4,300
U.S. treasury securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Long-term Investments
Money market funds	$ 4,300