Fair Value Measurements - Schedule of Fair Value Measurement (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	$ 1,170,675	$ 913,031
Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	225	222
Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	24,974
Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	874,191	737,861
U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	271,285	174,948
Total Fair Value
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	1,217,581	986,708
Other Assets:
Note receivable	23,245	22,879
Other Liabilities:
Contingent consideration obligation related to Velocius acquisition	(2,600)	(2,600)
Total Fair Value | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	21,401	40,482
Total Fair Value | Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	225	3,418
Total Fair Value | Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	48,471	29,999
Total Fair Value | Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	876,199	737,861
Total Fair Value | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	271,285	174,948
Level 1
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	21,626	43,900
Other Assets:
Note receivable	0	0
Other Liabilities:
Contingent consideration obligation related to Velocius acquisition	0	0
Level 1 | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	21,401	40,482
Level 1 | Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	225	3,418
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 2
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	1,195,955	942,808
Other Assets:
Note receivable	0	0
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	0
Level 2 | Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	48,471	29,999
Level 2 | Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	876,199	737,861
Level 2 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	271,285	174,948
Level 3
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	0	0
Other Assets:
Note receivable	23,245	22,879
Other Liabilities:
Contingent consideration obligation related to Velocius acquisition	(2,600)	(2,600)
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	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