Cash Equivalents and Marketable Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 381,742	$ 77,584
Debt Securities, Available-for-sale [Line Items]
Unrealized Gains	172	0
Unrealized Losses	(6)	(68)
Cash and cash equivalents and available-for-sale securities, amortized cost	466,519	72,900
Cash and cash equivalents and available-for-sale securities	466,685	72,832
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		7,448
Unrealized Gains		0
Unrealized Losses		0
Aggregate Fair Value		7,448
U.S. treasury debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,342	2,494
Unrealized Gains	8	0
Unrealized Losses	(1)	(1)
Aggregate Fair Value	10,349	2,493
U.S. corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	95,706	10,890
Unrealized Gains	164	0
Unrealized Losses	(5)	(67)
Aggregate Fair Value	95,865	10,823
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	360,471	52,068
Cash and cash equivalents, aggregate fair value	360,471	52,068
Cash and cash equivalents | Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	360,471	52,068
Cash and cash equivalents, aggregate fair value	360,471	52,068
Marketable securities | U.S. corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	106,048	20,832
Unrealized Gains	172	0
Unrealized Losses	(6)	(68)
Aggregate Fair Value	$ 106,214	$ 20,764