Fair Value Measurements - Cash Equivalents and Marketable Securities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	$ 765,177	$ 543,101
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	905,684
Total marketable securities	765,177
Total cash equivalents and marketable securities	1,670,861
Fair Value, Measurements, Recurring | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	905,684
Fair Value, Measurements, Recurring | U.S. Treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	85,251
Fair Value, Measurements, Recurring | Government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	101,976
Fair Value, Measurements, Recurring | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	577,950
Fair Value, Measurements, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	834,786
Total marketable securities	0
Total cash equivalents and marketable securities	834,786
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	834,786
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 1 | Government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 1 | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	70,898
Total marketable securities	765,177
Total cash equivalents and marketable securities	836,075
Fair Value, Measurements, Recurring | Level 2 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	70,898
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	85,251
Fair Value, Measurements, Recurring | Level 2 | Government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	101,976
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	577,950
Fair Value, Measurements, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0
Total marketable securities	0
Total cash equivalents and marketable securities	0
Fair Value, Measurements, Recurring | Level 3 | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash equivalents	0
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 3 | Government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	0
Fair Value, Measurements, Recurring | Level 3 | Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total marketable securities	$ 0