Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Cash and Cash Equivalents
Cash and cash equivalents	$ 1,995,482	$ 553,398
Treasury securities
Cash and Cash Equivalents
Cash and cash equivalents	25,022	44,052
Corporate debt securities
Cash and Cash Equivalents
Cash and cash equivalents	1,547,137	246,668
Level 1
Assets
Total cash equivalents and marketable securities	233,344	96,116
Liabilities
Convertible notes payable	0	0
Level 1 | Money market funds
Assets
Total cash equivalents and marketable securities	233,344	96,116
Level 1 | Government-backed securities
Assets
Total cash equivalents and marketable securities	0	0
Level 1 | Treasury securities
Assets
Total cash equivalents and marketable securities	0	0
Level 1 | Corporate debt securities
Assets
Total cash equivalents and marketable securities	0	0
Level 1 | Agency securities
Assets
Total cash equivalents and marketable securities	0	0
Level 2
Assets
Total cash equivalents and marketable securities	1,628,659	492,619
Liabilities
Convertible notes payable	533,705	407,238
Level 2 | Money market funds
Assets
Total cash equivalents and marketable securities	0	0
Level 2 | Government-backed securities
Assets
Total cash equivalents and marketable securities	54,250	44,250
Level 2 | Treasury securities
Assets
Total cash equivalents and marketable securities		54,088
Level 2 | Corporate debt securities
Assets
Total cash equivalents and marketable securities	1,549,387	373,681
Level 2 | Agency securities
Assets
Total cash equivalents and marketable securities	0	20,600
Level 3
Assets
Total cash equivalents and marketable securities	0	0
Liabilities
Convertible notes payable	0	0
Level 3 | Money market funds
Assets
Total cash equivalents and marketable securities	0	0
Level 3 | Government-backed securities
Assets
Total cash equivalents and marketable securities	0	0
Level 3 | Treasury securities
Assets
Total cash equivalents and marketable securities	0	0
Level 3 | Corporate debt securities
Assets
Total cash equivalents and marketable securities	0	0
Level 3 | Agency securities
Assets
Total cash equivalents and marketable securities	$ 0	$ 0