Fair Value Measurements - Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	$ 1,391,721	$ 912,149
Gross unrealized gains	474	1,248
Gross unrealized losses	(2,561)	(366)
Available-for-sale securities	1,389,634us-gaap_AvailableForSaleSecurities	913,031us-gaap_AvailableForSaleSecurities
Short-term marketable securities	519,642	340,005
Long-term marketable securities	869,992	573,026
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	39	222
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Available-for-sale securities	39	222
Short-term marketable securities	0	173
Long-term marketable securities	39	49
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	10,487
Gross unrealized gains	0
Gross unrealized losses	(2)
Available-for-sale securities	10,485
Short-term marketable securities	10,485
Long-term marketable securities	0
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	1,077,387	736,945
Gross unrealized gains	454	1,197
Gross unrealized losses	(2,132)	(281)
Available-for-sale securities	1,075,709	737,861
Short-term marketable securities	424,777	278,318
Long-term marketable securities	650,932	459,543
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	303,808	174,982
Gross unrealized gains	20	51
Gross unrealized losses	(427)	(85)
Available-for-sale securities	303,401	174,948
Short-term marketable securities	84,380	61,514
Long-term marketable securities	$ 219,021	$ 113,434

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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