Financial Instruments - Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2020 USD ($) option	Dec. 31, 2019 USD ($) option
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Losses, Less than 12 months	$ (509)	$ (40)
Estimated Fair Value, Less than 12 months	759,483	85,161
Gross Unrealized Losses, 12 months or more	0	0
Estimated Fair Value, 12 months or more	0	0
Gross Unrealized Losses, Total	(509)	(40)
Estimated Fair Value, Total	$ 759,483	$ 85,161
Number of AFS securities in loss positions | option	0	19
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Losses, Less than 12 months	$ (1)	$ (6)
Estimated Fair Value, Less than 12 months	14,893	12,822
Gross Unrealized Losses, 12 months or more	0	0
Estimated Fair Value, 12 months or more	0	0
Gross Unrealized Losses, Total	(1)	(6)
Estimated Fair Value, Total	14,893	12,822
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Losses, Less than 12 months	(1)	(2)
Estimated Fair Value, Less than 12 months	15,341	9,979
Gross Unrealized Losses, 12 months or more	0	0
Estimated Fair Value, 12 months or more	0	0
Gross Unrealized Losses, Total	(1)	(2)
Estimated Fair Value, Total	15,341	9,979
Debt securities of U.S. government agencies and corporate entities
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Losses, Less than 12 months	(507)	(32)
Estimated Fair Value, Less than 12 months	729,249	62,360
Gross Unrealized Losses, 12 months or more	0	0
Estimated Fair Value, 12 months or more	0	0
Gross Unrealized Losses, Total	(507)	(32)
Estimated Fair Value, Total	$ 729,249	$ 62,360