Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 520,374	$ 1,059,525
Gross Unrealized Gains	24	1
Gross Unrealized Losses	(2,009)	(4,770)
Aggregate Fair Value	518,389	1,054,756
Short-Term Marketable Securities	429,932	855,499
Long-Term Marketable Securities	88,457	199,257
Unrealized losses included in accumulated other comprehensive loss	461,700
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		40,000
Gross Unrealized Gains		0
Gross Unrealized Losses		(7)
Aggregate Fair Value		39,993
Short-Term Marketable Securities		39,993
Long-Term Marketable Securities		0
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,249	282,996
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(50)
Aggregate Fair Value	1,249	282,946
Short-Term Marketable Securities	1,249	282,946
Long-Term Marketable Securities	0	0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	468,247	685,653
Gross Unrealized Gains	21	1
Gross Unrealized Losses	(1,799)	(4,309)
Aggregate Fair Value	466,469	681,345
Short-Term Marketable Securities	378,012	482,088
Long-Term Marketable Securities	88,457	199,257
Available-for-sale marketable securities, continuous unrealized loss position for more than 12 months	(1,900)
U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,878	50,876
Gross Unrealized Gains	3	0
Gross Unrealized Losses	(210)	(404)
Aggregate Fair Value	50,671	50,472
Short-Term Marketable Securities	50,671	50,472
Long-Term Marketable Securities	$ 0	$ 0

Available-for-sale Securities, Current, Excluding Restricted Trading Securities

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- Definition

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

+ References

No definition available.

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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=82897076&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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No definition available.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

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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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