Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 959,302	$ 963,867
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(9,157)	(5,602)
Aggregate Fair Value	950,145	958,267
Short-Term Marketable Securities	447,795	398,554
Long-Term Marketable Securities	502,350	559,713
Unrealized losses included in accumulated other comprehensive loss	558,700
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,707	6,951
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(17)	(9)
Aggregate Fair Value	5,690	6,942
Short-Term Marketable Securities	5,690	6,942
Long-Term Marketable Securities	0	0
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	734,080	736,902
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(7,253)	(3,829)
Aggregate Fair Value	726,827	733,075
Short-Term Marketable Securities	301,960	289,378
Long-Term Marketable Securities	424,867	443,697
Available-for-sale marketable securities, continuous unrealized loss position for more than 12 months	4,900
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	219,515	220,014
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,887)	(1,764)
Aggregate Fair Value	217,628	218,250
Short-Term Marketable Securities	140,145	102,234
Long-Term Marketable Securities	$ 77,483	$ 116,016

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.

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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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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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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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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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