Fair Value Measurements - Summary of Fair Value Measurement of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 627,020	$ 393,968
Unrealized Gain	332	98
Unrealized Loss	(31)	(160)
Fair Value	627,321	393,906
Cash equivalents (due within 90 days)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	465,257	245,194
Fair Value	465,257	245,194
Short-term investments (due within one year)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	92,522	68,656
Unrealized Gain	49
Unrealized Loss	(31)	(92)
Fair Value	92,540	68,564
Long-term investments (due between one and two years)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	69,241	80,118
Unrealized Gain	283	98
Unrealized Loss		(68)
Fair Value	$ 69,524	$ 80,148

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

No definition available.

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- Definition

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=82897076&loc=d3e27161-111563

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- Definition

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

+ References

No definition available.

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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- Definition

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.

+ References

No definition available.

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