Short-Term Investments - Summary (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 490,673	$ 368,146
Gross unrealized gains	878	375
Gross unrealized losses	(696)	(6)
Estimated fair value	$ 490,855	$ 368,515
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Maturity	2 years	2 years
Amortized cost	$ 228,256	$ 160,065
Gross unrealized gains	162	233
Gross unrealized losses	(692)	(1)
Estimated fair value	$ 227,726	$ 160,297
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Maturity	1 year	1 year
Amortized cost	$ 192,594	$ 120,862
Gross unrealized gains	440	74
Gross unrealized losses	(4)	0
Estimated fair value	$ 193,030	$ 120,936
U.S. Agency bonds
Debt Securities, Available-for-sale [Line Items]
Maturity	2 years	2 years
Amortized cost	$ 44,745	$ 50,745
Gross unrealized gains	122	41
Gross unrealized losses	0	(4)
Estimated fair value	$ 44,867	$ 50,782
U.S. Treasury bills
Debt Securities, Available-for-sale [Line Items]
Maturity	1 year	2 years
Amortized cost	$ 25,078	$ 36,474
Gross unrealized gains	154	27
Gross unrealized losses	0	(1)
Estimated fair value	$ 25,232	$ 36,500

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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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Amount, before tax, of unrealized loss 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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Amortized cost of 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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