Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Cash	$ 11,271	$ 43,058
Total cash and cash equivalents	269,975	62,070
Marketable securities, amortized cost	150,011	188,706
Marketable securities, gross unrealized gains	273	381
Marketable securities, gross unrealized losses	(66)	(2)
Marketable securities, estimated fair value	150,218	189,085
Total cash, cash equivalents and marketable securities, amortized cost	419,986	250,776
Total cash, cash equivalents and marketable securities	420,193	251,155
Amortized Cost
Due within one year, amortized cost	127,968
Due between one and two years, amortized cost	22,043
Marketable securities, amortized cost	150,011
Fair Value
Due within one year, estimated fair value	128,147
Due between one and two years, estimated fair value	22,071
Marketable securities, estimated fair value	150,218
Prepaid expenses and other current assets
Fair Value
Accrued interest receivable	700	600
Commercial paper
Cash and Cash Equivalents [Line Items]
Cash equivalents	34,634	7,485
Marketable securities, amortized cost	39,528	73,366
Marketable securities, gross unrealized gains	0	0
Marketable securities, gross unrealized losses	0	0
Marketable securities, estimated fair value	39,528	73,366
Money market funds
Cash and Cash Equivalents [Line Items]
Cash equivalents	224,070	11,527
Corporate debt securities
Cash and Cash Equivalents [Line Items]
Marketable securities, amortized cost	70,543	74,038
Marketable securities, gross unrealized gains	273	381
Marketable securities, gross unrealized losses	(66)	(2)
Marketable securities, estimated fair value	70,750	74,417
Certificate of deposits
Cash and Cash Equivalents [Line Items]
Marketable securities, amortized cost	39,940	41,302
Marketable securities, gross unrealized gains	0	0
Marketable securities, gross unrealized losses	0	0
Marketable securities, estimated fair value	$ 39,940	$ 41,302