Fair Value Measurements - Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,348,486	$ 912,149
Gross Unrealized Gains	908	1,248
Gross Unrealized Losses	(1,974)	(366)
Aggregate Fair Value	1,347,420	913,031
Short-Term Marketable Securities	496,573	340,005
Long-Term Marketable Securities	850,847	573,026
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	52	222
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Aggregate Fair Value	52	222
Short-Term Marketable Securities	0	173
Long-Term Marketable Securities	52	49
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,487
Gross Unrealized Gains	1
Gross Unrealized Losses	(2)
Aggregate Fair Value	22,486
Short-Term Marketable Securities	22,486
Long-Term Marketable Securities	0
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,032,801	736,945
Gross Unrealized Gains	870	1,197
Gross Unrealized Losses	(1,646)	(281)
Aggregate Fair Value	1,032,025	737,861
Short-Term Marketable Securities	399,682	278,318
Long-Term Marketable Securities	632,343	459,543
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	293,146	174,982
Gross Unrealized Gains	37	51
Gross Unrealized Losses	(326)	(85)
Aggregate Fair Value	292,857	174,948
Short-Term Marketable Securities	74,405	61,514
Long-Term Marketable Securities	$ 218,452	$ 113,434

- 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.

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