Fair Value Measurements - Schedule of Fair Value Measurement (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Equivalents and Marketable Securities:
Available-for-sale securities	$ 1,963,456	$ 1,054,756
Assets, fair value	2,029,412	1,445,032
Liabilities:
Contingent consideration obligation related to completed acquisitions	0	(6,300)	$ (8,631)
Money market funds
Cash Equivalents and Marketable Securities:
Cash equivalents	50,779	380,260
Certificates of deposit
Cash Equivalents and Marketable Securities:
Available-for-sale securities	150,000	39,993
Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	73,845	282,946
Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	1,370,130	681,345
U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	369,481	50,472
Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	15,177	10,016
Level 1
Cash Equivalents and Marketable Securities:
Assets, fair value	65,956	430,269
Liabilities:
Contingent consideration obligation related to completed acquisitions		0
Level 1 | Money market funds
Cash Equivalents and Marketable Securities:
Cash equivalents	50,779	380,260
Level 1 | Certificates of deposit
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	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	15,177	10,016
Level 2
Cash Equivalents and Marketable Securities:
Assets, fair value	1,963,456	1,014,763
Liabilities:
Contingent consideration obligation related to completed acquisitions		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	150,000	0
Level 2 | Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	73,845	282,946
Level 2 | Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	1,370,130	681,345
Level 2 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	369,481	50,472
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)
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	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