Summary of Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 553,398	$ 78,823
Cash
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	122,312	15,863
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	96,116	42,960
Government-backed securities
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	44,250	20,000
Treasury securities
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	44,052	0
Corporate debt securities
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 246,668	$ 0