Fair Value Measurements - Schedule of Fair Value Measurement (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	$ 1,342,175	$ 958,267
Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	64,994
Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	393,740	6,942
Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	825,322	733,075
U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	58,119	218,250
Level 1
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	215,312	30,528
Contingent consideration related to a completed acquisition	0	0
Level 1 | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	139,125	22,649
Level 1 | Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	64,994
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	11,193	7,879
Level 2
Cash Equivalents and Marketable Securities:
Cash equivalents and marketable securities	1,277,181	962,251
Contingent consideration related to a completed 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
Level 2 | Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	393,740	10,928
Level 2 | Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	825,322	733,075
Level 2 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	58,119	218,248
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 related to a completed acquisition	(6,200)	(8,631)
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	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	1,492,493	992,779
Contingent consideration related to a completed acquisition	(6,200)	(8,631)
Total Fair Value | Money market funds
Cash Equivalents and Marketable Securities:
Money market funds	139,125	22,649
Total Fair Value | Certificates of deposit
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	64,994
Total Fair Value | Commercial paper
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	393,740	10,928
Total Fair Value | Corporate bonds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	825,322	733,075
Total Fair Value | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	58,119	218,248
Total Fair Value | Mutual funds
Cash Equivalents and Marketable Securities:
Aggregate Fair Value	$ 11,193	$ 7,879