Fair Value Measurements - Marketable Securities (Details) $ in Thousands	Sep. 30, 2021 USD ($) corporateBonds	Dec. 31, 2020 USD ($)
Marketable Securities [Line Items]
Amortized Cost	$ 1,462,703	$ 2,114,541
Gross Unrealized Gains	3,146	9,824
Gross Unrealized Losses	(509)	(339)
Aggregate Fair Value	1,465,340	2,124,026
Short-Term Marketable Securities	651,926	744,290
Long-Term Marketable Securities	813,414	1,379,736
Commercial paper
Marketable Securities [Line Items]
Amortized Cost	79,744	46,931
Gross Unrealized Gains	2	13
Gross Unrealized Losses	(9)	(8)
Aggregate Fair Value	79,737	46,936
Short-Term Marketable Securities	79,737	46,936
Long-Term Marketable Securities	0	0
Corporate bonds
Marketable Securities [Line Items]
Amortized Cost	1,097,056	1,628,462
Gross Unrealized Gains	3,100	9,482
Gross Unrealized Losses	(378)	(262)
Aggregate Fair Value	1,099,778	1,637,682
Short-Term Marketable Securities	507,238	607,403
Long-Term Marketable Securities	$ 592,540	1,030,279
Number of corporate bond investments classified as available-for-sale marketable securities in an unrealized loss position | corporateBonds	1
Available-for-sale marketable securities, continuous unrealized loss position for more than 12 months	$ 1,500
U.S. government agency obligations
Marketable Securities [Line Items]
Amortized Cost	285,903	435,653
Gross Unrealized Gains	44	329
Gross Unrealized Losses	(122)	(63)
Aggregate Fair Value	285,825	435,919
Short-Term Marketable Securities	64,951	89,951
Long-Term Marketable Securities	$ 220,874	345,968
Municipal securities
Marketable Securities [Line Items]
Amortized Cost		3,495
Gross Unrealized Gains		0
Gross Unrealized Losses		(6)
Aggregate Fair Value		3,489
Short-Term Marketable Securities		0
Long-Term Marketable Securities		$ 3,489