Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, cost	$ 1,059,525	$ 963,867
Gross unrealized gains	1	2
Gross unrealized losses	(4,770)	(5,602)
Available-for-sale securities	1,054,756	958,267
Short-term marketable securities	855,499	398,554
Long-term marketable securities	199,257	559,713
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, cost	40,000
Gross unrealized gains	0
Gross unrealized losses	(7)
Available-for-sale securities	39,993
Short-term marketable securities	39,993
Long-term marketable securities	0
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, cost	282,996	6,951
Gross unrealized gains	0	0
Gross unrealized losses	(50)	(9)
Available-for-sale securities	282,946	6,942
Short-term marketable securities	282,946	6,942
Long-term marketable securities	0	0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, cost	685,653	736,902
Gross unrealized gains	1	2
Gross unrealized losses	(4,309)	(3,829)
Available-for-sale securities	681,345	733,075
Short-term marketable securities	482,088	289,378
Long-term marketable securities	199,257	443,697
U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, cost	50,876	220,014
Gross unrealized gains	0	0
Gross unrealized losses	(404)	(1,764)
Available-for-sale securities	50,472	218,250
Short-term marketable securities	50,472	102,234
Long-term marketable securities	$ 0	$ 116,016

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.

+ Details

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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

No definition available.

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- Definition

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.

+ References

No definition available.

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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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