Financial Instruments - Cash Equivalents, Restricted Cash and Investments, Classified as Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 537,235	$ 535,042
Unrealized Gains	2,330	1,595
Unrealized Losses	(277)	(8)
Cash equivalents	122,141	122,900
Short-term investments	357,026	355,407
Long-term investments	60,121	58,322
Total cash equivalents and investments	539,288	536,629
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	113,143	122,900
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	113,143	122,900
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,786	204,144
Unrealized Gains	0	871
Unrealized Losses	0	(4)
Fair Value	25,786	205,011
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	190,625	181,340
Unrealized Gains	505	557
Unrealized Losses	(276)	(3)
Fair Value	190,854	181,894
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	178,695	25,658
Unrealized Gains	1,618	167
Unrealized Losses	0	(1)
Fair Value	180,313	25,824
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	28,986
Unrealized Gains	207
Unrealized Losses	(1)
Fair Value	$ 29,192
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		1,000
Unrealized Gains		0
Unrealized Losses		0
Fair Value		$ 1,000

Cash equivalents and short term investments.

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- Definition

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).

+ References

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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).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 2
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Amount 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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-Publisher FASB
-Name Accounting Standards Codification
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (available-for-sale), classified as noncurrent.

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Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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