Short-term Investments - Summary (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 233,167	$ 190,137
Gross unrealized gains	275	26
Gross unrealized losses	(7)	(67)
Estimated fair value	$ 233,435	$ 190,096
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Maturity	2 years	1 year
Amortized cost	$ 116,958	$ 111,933
Gross unrealized gains	156	26
Gross unrealized losses	(5)	(43)
Estimated fair value	$ 117,109	$ 111,916
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Maturity	1 year	1 year
Amortized cost	$ 89,293	$ 74,433
Gross unrealized gains	93	0
Gross unrealized losses	0	(24)
Estimated fair value	$ 89,386	$ 74,409
U.S. Treasury bills
Debt Securities, Available-for-sale [Line Items]
Maturity	1 year	1 year
Amortized cost	$ 18,921	$ 3,771
Gross unrealized gains	25	0
Gross unrealized losses	0	0
Estimated fair value	$ 18,946	$ 3,771
U.S. Agency bonds
Debt Securities, Available-for-sale [Line Items]
Maturity	2 years
Amortized cost	$ 6,000
Gross unrealized gains	0
Gross unrealized losses	(2)
Estimated fair value	$ 5,998
Other sovereign securities
Debt Securities, Available-for-sale [Line Items]
Maturity	1 year
Amortized cost	$ 1,995
Gross unrealized gains	1
Gross unrealized losses	0
Estimated fair value	$ 1,996

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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