Investments - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 368,146	$ 190,137
Gross unrealized gains	375	26
Gross unrealized losses	(6)	(67)
Estimated fair value	$ 368,515	$ 190,096
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Maturity (or less)	2 years	1 year
Amortized cost	$ 160,065	$ 111,933
Gross unrealized gains	233	26
Gross unrealized losses	(1)	(43)
Estimated fair value	$ 160,297	$ 111,916
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Maturity (or less)	1 year	1 year
Amortized cost	$ 120,862	$ 74,433
Gross unrealized gains	74	0
Gross unrealized losses	0	(24)
Estimated fair value	$ 120,936	$ 74,409
U.S. Agency Bonds
Debt Securities, Available-for-sale [Line Items]
Maturity (or less)	2 years
Amortized cost	$ 50,745
Gross unrealized gains	41
Gross unrealized losses	(4)
Estimated fair value	$ 50,782
U.S. Treasury bills
Debt Securities, Available-for-sale [Line Items]
Maturity (or less)	2 years	1 year
Amortized cost	$ 36,474	$ 3,771
Gross unrealized gains	27	0
Gross unrealized losses	(1)	0
Estimated fair value	$ 36,500	$ 3,771

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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