Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost	$ 471,840	$ 265,109
Gross Unrealized Holding Loss	(71)	(44)
Gross Unrealized Holding Gain	286	12
Fair Value	472,055	265,077
Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Cost	373,762	257,334
Gross Unrealized Holding Loss	(34)	(44)
Gross Unrealized Holding Gain	231	12
Fair Value	373,959	257,302
Short-term Investments \| Commercial paper
Debt Securities, Available-for-sale [Line Items]
Cost	29,319	147,860
Gross Unrealized Holding Loss	(1)	0
Gross Unrealized Holding Gain	2	0
Fair Value	29,320	147,860
Short-term Investments \| Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Cost	26,129
Gross Unrealized Holding Loss	(3)
Gross Unrealized Holding Gain	6
Fair Value	26,132
Short-term Investments \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Cost	114,068	46,836
Gross Unrealized Holding Loss	(22)	(35)
Gross Unrealized Holding Gain	156	0
Fair Value	114,202	46,801
Short-term Investments \| U.S. Government and agency securities
Debt Securities, Available-for-sale [Line Items]
Cost	204,246	62,638
Gross Unrealized Holding Loss	(8)	(9)
Gross Unrealized Holding Gain	67	12
Fair Value	204,305	62,641
Long-term Investments
Debt Securities, Available-for-sale [Line Items]
Cost	89,326
Gross Unrealized Holding Loss	(37)
Gross Unrealized Holding Gain	54
Fair Value	89,343
Long-term Investments \| Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Cost	4,727
Gross Unrealized Holding Loss	0
Gross Unrealized Holding Gain	2
Fair Value	4,729
Long-term Investments \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Cost	38,582
Gross Unrealized Holding Loss	(35)
Gross Unrealized Holding Gain	16
Fair Value	38,563
Long-term Investments \| U.S. Government and agency securities
Debt Securities, Available-for-sale [Line Items]
Cost	46,017
Gross Unrealized Holding Loss	(2)
Gross Unrealized Holding Gain	36
Fair Value	46,051
Cash Equivalents
Debt Securities, Available-for-sale [Line Items]
Cost	8,752	7,775
Gross Unrealized Holding Loss	0	0
Gross Unrealized Holding Gain	1	0
Fair Value	8,753	7,775
Cash Equivalents \| Commercial paper
Debt Securities, Available-for-sale [Line Items]
Cost	5,794	7,775
Gross Unrealized Holding Loss	0	0
Gross Unrealized Holding Gain	0	0
Fair Value	5,794	$ 7,775
Cash Equivalents \| Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Cost	2,958
Gross Unrealized Holding Loss	0
Gross Unrealized Holding Gain	1
Fair Value	$ 2,959

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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