Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Marketable Securities [Line Items]
Amortized Cost	$ 1,973,512	$ 2,114,541
Gross Unrealized Gains	5,063	9,824
Gross Unrealized Losses	(1,065)	(339)
Aggregate Fair Value	1,977,510	2,124,026
Short-Term Marketable Securities	835,086	744,290
Long-Term Marketable Securities	1,142,424	1,379,736
Commercial paper
Marketable Securities [Line Items]
Amortized Cost	77,775	46,931
Gross Unrealized Gains	8	13
Gross Unrealized Losses	(1)	(8)
Aggregate Fair Value	77,782	46,936
Short-Term Marketable Securities	77,782	46,936
Long-Term Marketable Securities	0	0
Corporate bonds
Marketable Securities [Line Items]
Amortized Cost	1,529,918	1,628,462
Gross Unrealized Gains	4,965	9,482
Gross Unrealized Losses	(760)	(262)
Aggregate Fair Value	1,534,123	1,637,682
Short-Term Marketable Securities	673,847	607,403
Long-Term Marketable Securities	860,276	1,030,279
U.S. government agency obligations
Marketable Securities [Line Items]
Amortized Cost	365,819	435,653
Gross Unrealized Gains	90	329
Gross Unrealized Losses	(304)	(63)
Aggregate Fair Value	365,605	435,919
Short-Term Marketable Securities	83,457	89,951
Long-Term Marketable Securities	$ 282,148	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