Investments - Summary (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Mar. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost		$ 446,679	$ 368,146
Gross unrealized gains		940	375
Gross unrealized losses		(3)	(6)
Estimated fair value		$ 447,616	368,515
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Maturity	2 years	2 years
Amortized cost		$ 159,588	160,065
Gross unrealized gains		576	233
Gross unrealized losses		0	(1)
Estimated fair value		$ 160,164	160,297
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Maturity	1 year	1 year
Amortized cost		$ 154,795	120,862
Gross unrealized gains		296	74
Gross unrealized losses		0	0
Estimated fair value		$ 155,091	120,936
U.S. Agency bonds
Debt Securities, Available-for-sale [Line Items]
Maturity	2 years	2 years
Amortized cost		$ 79,120	50,745
Gross unrealized gains		38	41
Gross unrealized losses		(3)	(4)
Estimated fair value		$ 79,155	50,782
U.S. Treasury bills
Debt Securities, Available-for-sale [Line Items]
Maturity	2 years	1 year
Amortized cost		$ 53,176	36,474
Gross unrealized gains		30	27
Gross unrealized losses		0	(1)
Estimated fair value		$ 53,206	$ 36,500

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