Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financial Assets:
Marketable securities	$ 1,296,261	$ 176,674
Fair Value, Recurring
Financial Assets:
Total financial assets	1,476,897	765,436
Fair Value, Recurring | Money Market Funds
Financial Assets:
Cash equivalents	159,536	588,762
Fair Value, Recurring | Commercial Paper
Financial Assets:
Cash equivalents	10,500
Marketable securities	125,240	23,784
Fair Value, Recurring | Corporate Debt Securities
Financial Assets:
Cash equivalents	10,601
Marketable securities	967,028	80,417
Fair Value, Recurring | Certificates of Deposit
Financial Assets:
Marketable securities	34,261
Fair Value, Recurring | U.S. Government Treasury Securities
Financial Assets:
Marketable securities	169,731	72,473
Fair Value, Recurring | Level 1
Financial Assets:
Total financial assets	159,536	588,762
Fair Value, Recurring | Level 1 | Money Market Funds
Financial Assets:
Cash equivalents	159,536	588,762
Fair Value, Recurring | Level 2
Financial Assets:
Total financial assets	1,317,361	176,674
Fair Value, Recurring | Level 2 | Commercial Paper
Financial Assets:
Cash equivalents	10,500
Marketable securities	125,240	23,784
Fair Value, Recurring | Level 2 | Corporate Debt Securities
Financial Assets:
Cash equivalents	10,601
Marketable securities	967,028	80,417
Fair Value, Recurring | Level 2 | Certificates of Deposit
Financial Assets:
Marketable securities	34,261
Fair Value, Recurring | Level 2 | U.S. Government Treasury Securities
Financial Assets:
Marketable securities	$ 169,731	$ 72,473