Fair Value Measurements - Schedule of Fair Value Measurement (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Equivalents and Marketable Securities:
Available-for-sale securities	$ 1,054,756	$ 958,267
Certificates of deposit
Cash Equivalents and Marketable Securities:
Available-for-sale securities	39,993
Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	282,946	6,942
Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	681,345	733,075
U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	50,472	218,250
Level 1
Cash Equivalents and Marketable Securities:
Assets, fair value	430,269	30,528
Liabilities:
Contingent consideration obligation related to completed acquisitions	0	0
Level 1 | Money market funds
Cash Equivalents and Marketable Securities:
Cash equivalents	380,260	22,649
Level 1 | Certificates of deposit
Cash Equivalents and Marketable Securities:
Available-for-sale securities	39,993
Level 1 | Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 1 | Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 1 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 1 | Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	10,016	7,879
Level 2
Cash Equivalents and Marketable Securities:
Assets, fair value	1,014,763	962,251
Liabilities:
Contingent consideration obligation related to completed acquisitions	0	0
Level 2 | Money market funds
Cash Equivalents and Marketable Securities:
Cash equivalents	0	0
Level 2 | Certificates of deposit
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0
Level 2 | Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	282,946	10,928
Level 2 | Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	681,345	733,075
Level 2 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	50,472	218,248
Level 2 | Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 3
Cash Equivalents and Marketable Securities:
Assets, fair value	0	0
Liabilities:
Contingent consideration obligation related to completed acquisitions	(6,300)	(8,631)	$ (7,100)
Level 3 | Money market funds
Cash Equivalents and Marketable Securities:
Cash equivalents	0	0
Level 3 | Certificates of deposit
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0
Level 3 | Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 3 | Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 3 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 3 | Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Total Fair Value
Cash Equivalents and Marketable Securities:
Assets, fair value	1,445,032	992,779
Total Fair Value | Money market funds
Cash Equivalents and Marketable Securities:
Cash equivalents	380,260	22,649
Total Fair Value | Certificates of deposit
Cash Equivalents and Marketable Securities:
Available-for-sale securities	39,993
Total Fair Value | Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	282,946	10,928
Total Fair Value | Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	681,345	733,075
Total Fair Value | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	50,472	218,248
Total Fair Value | Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	$ 10,016	$ 7,879