Fair Value Measurements - Schedule of Fair Value Measurement (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	$ 1,182,195,000us-gaap_AvailableForSaleSecurities	$ 1,389,634,000us-gaap_AvailableForSaleSecurities
Other Liabilities:
Maximum potential payout of the contingent consideration	1,000,000
Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value		39,000
Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	10,493,000	10,485,000
Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	932,175,000	1,075,709,000
U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	239,527,000	303,401,000
Level 1
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	1,259,000	540,000
Other Liabilities:
Contingent consideration obligation related to Velocius acquisition	0	0
Level 1 \| Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	872,000	501,000
Level 1 \| Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value		39,000
Level 1 \| Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	0	0
Level 1 \| Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	0	0
Level 1 \| U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	0	0
Level 1 \| Mutual funds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	387,000
Level 2
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	1,184,195,000	1,389,595,000
Other Liabilities:
Contingent consideration obligation related to Velocius acquisition	0	0
Level 2 \| Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	0	0
Level 2 \| Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value		0
Level 2 \| Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	12,493,000	10,485,000
Level 2 \| Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	932,175,000	1,075,709,000
Level 2 \| U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	239,527,000	303,401,000
Level 2 \| Mutual funds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	0
Level 3
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	0	0
Other Liabilities:
Contingent consideration obligation related to Velocius acquisition	(900,000)	(900,000)
Level 3 \| Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	0	0
Level 3 \| Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value		0
Level 3 \| Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	0	0
Level 3 \| Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	0	0
Level 3 \| U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	0	0
Level 3 \| Mutual funds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	0
Total Fair Value
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	1,185,454,000	1,390,135,000
Other Liabilities:
Contingent consideration obligation related to Velocius acquisition	(900,000)	(900,000)
Total Fair Value \| Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	872,000	501,000
Total Fair Value \| Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value		39,000
Total Fair Value \| Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	12,493,000	10,485,000
Total Fair Value \| Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	932,175,000	1,075,709,000
Total Fair Value \| U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	239,527,000	303,401,000
Total Fair Value \| Mutual funds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	$ 387,000

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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

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For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value of obligations measured on a recurring basis.

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