Fair Value Measurements - Schedule of Fair Value Measurement (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	$ 1,247,355	$ 1,389,634
Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value		39
Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value		10,485
Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	1,016,525	1,075,709
U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	230,830	303,401
Level 1
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	2,852	540
Other Liabilities:
Contingent consideration obligation related to Velocius acquisition		0
Level 1 | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	1,805	501
Level 1 | Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value		39
Level 1 | Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value		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	1,047
Level 2
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	1,247,355	1,389,595
Other Liabilities:
Contingent consideration obligation related to Velocius acquisition		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		10,485
Level 2 | Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	1,016,525	1,075,709
Level 2 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	230,830	303,401
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)
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
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	1,250,207	1,390,135
Other Liabilities:
Contingent consideration obligation related to Velocius acquisition		(900)
Total Fair Value | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	1,805	501
Total Fair Value | Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value		39
Total Fair Value | Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value		10,485
Total Fair Value | Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	1,016,525	1,075,709
Total Fair Value | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	230,830	$ 303,401
Total Fair Value | Mutual funds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	$ 1,047