Fair Value Measurements - Schedule of Fair Value Measurement (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Cash Equivalents and Marketable Securities:
Available-for-sale securities	$ 2,124,026	$ 1,963,456
Total Fair Value	2,247,224	2,029,412
Money market funds
Cash Equivalents and Marketable Securities:
Cash equivalents	74,417	50,779
Certificates of deposit
Cash Equivalents and Marketable Securities:
Available-for-sale securities		150,000
Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	46,936	73,845
Available-for-sale securities and cash equivalents	75,785
Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	1,637,682	1,370,130
Municipal securities
Cash Equivalents and Marketable Securities:
Available-for-sale securities	3,489
U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	435,919	369,481
Available-for-sale securities and cash equivalents	435,919
Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	19,932	15,177
Level 1
Cash Equivalents and Marketable Securities:
Total Fair Value	94,349	65,956
Level 1 | Money market funds
Cash Equivalents and Marketable Securities:
Cash equivalents	74,417	50,779
Level 1 | Certificates of deposit
Cash Equivalents and Marketable Securities:
Available-for-sale securities		0
Level 1 | Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities		0
Available-for-sale securities and cash equivalents	0
Level 1 | Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 1 | Municipal securities
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0
Level 1 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities		0
Available-for-sale securities and cash equivalents	0
Level 1 | Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	19,932	15,177
Level 2
Cash Equivalents and Marketable Securities:
Total Fair Value	2,152,875	1,963,456
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
Level 2 | Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities		73,845
Available-for-sale securities and cash equivalents	75,785
Level 2 | Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	1,637,682	1,370,130
Level 2 | Municipal securities
Cash Equivalents and Marketable Securities:
Available-for-sale securities	3,489
Level 2 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities		369,481
Available-for-sale securities and cash equivalents	435,919
Level 2 | Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 3
Cash Equivalents and Marketable Securities:
Total Fair Value	0	0
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
Available-for-sale securities and cash equivalents	0
Level 3 | Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 3 | Municipal securities
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0
Level 3 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities		0
Available-for-sale securities and cash equivalents	0
Level 3 | Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	$ 0	$ 0