Fair Value Measurements - Schedule of Fair Value Measurement (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	$ 1,207,701	$ 1,233,348
Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	2,494	2,487
Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	963,629	991,808
U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	241,578	239,053
Level 1
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	24,654	2,664
Level 1 | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	22,035	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	2,619	1,414
Level 2
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	1,207,701	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	2,494	2,487
Level 2 | Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	963,629	991,808
Level 2 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	241,578	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	1,232,355	1,236,012
Total Fair Value | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	22,035	1,250
Total Fair Value | Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	2,494	2,487
Total Fair Value | Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	963,629	991,808
Total Fair Value | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	241,578	239,053
Total Fair Value | Mutual funds
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	$ 2,619	$ 1,414