Fair Value Measurements - Schedule of Fair Value Measurement (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	$ 1,038,683	$ 1,288,138
Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	5,953	40,920
Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	813,897	981,076
U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	218,833	266,142
Level 1
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	33,298	12,748
Contingent consideration obligations related to completed acquisitions	0	0
Level 1 | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	26,201	8,726
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	7,097	4,022
Level 2
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	1,038,683	1,288,138
Contingent consideration obligations related to completed acquisitions	0	0
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	5,953	40,920
Level 2 | Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	813,897	981,076
Level 2 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	218,833	266,142
Level 2 | Mutual funds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	0	0
Level 3
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	0	0
Contingent consideration obligations related to completed acquisitions	(5,300)	(7,100)
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	0
Total Fair Value
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	1,071,981	1,300,886
Contingent consideration obligations related to completed acquisitions	(5,300)	(7,100)
Total Fair Value | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	26,201	8,726
Total Fair Value | Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	5,953	40,920
Total Fair Value | Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	813,897	981,076
Total Fair Value | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	218,833	266,142
Total Fair Value | Mutual funds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	$ 7,097	$ 4,022