Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	$ 963,867	$ 1,293,088
Gross unrealized gains	2	158
Gross unrealized losses	(5,602)	(5,108)
Available-for-sale securities	958,267	1,288,138
Short-term marketable securities	398,554	512,572
Long-term marketable securities	559,713	775,566
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	6,951	40,965
Gross unrealized gains	0	0
Gross unrealized losses	(9)	(45)
Available-for-sale securities	6,942	40,920
Short-term marketable securities	6,942	40,920
Long-term marketable securities	0	0
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	736,902	984,650
Gross unrealized gains	2	123
Gross unrealized losses	(3,829)	(3,697)
Available-for-sale securities	733,075	981,076
Short-term marketable securities	289,378	418,495
Long-term marketable securities	443,697	562,581
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	220,014	267,473
Gross unrealized gains	0	35
Gross unrealized losses	(1,764)	(1,366)
Available-for-sale securities	218,250	266,142
Short-term marketable securities	102,234	53,157
Long-term marketable securities	$ 116,016	$ 212,985

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

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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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.

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-Publisher FASB
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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.

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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.

+ References

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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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