Fair Value Measurements - Schedule of Fair Value Measurement (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	$ 1,270,280	$ 1,233,348
Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	18,363	2,487
Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	981,866	991,808
U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	270,051	239,053
Level 1
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	6,803	2,664
Level 1 | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	3,789	1,250
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:
Cash equivalents and marketable securities	3,014	1,414
Level 2
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	1,270,280	1,233,348
Level 2 | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	0	0
Level 2 | Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	18,363	2,487
Level 2 | Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	981,866	991,808
Level 2 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	270,051	239,053
Level 2 | Mutual funds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	0
Cash equivalents and marketable securities		0
Level 3
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	0	0
Level 3 | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	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
Level 3 | Mutual funds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	0
Cash equivalents and marketable securities		0
Total Fair Value
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	1,277,083	1,236,012
Total Fair Value | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	3,789	1,250
Total Fair Value | Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	18,363	2,487
Total Fair Value | Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	981,866	991,808
Total Fair Value | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	270,051	239,053
Total Fair Value | Mutual funds
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	$ 3,014	$ 1,414