Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Marketable Securities [Line Items]
Amortized Cost	$ 1,970,001	$ 2,114,541
Gross Unrealized Gains	6,728	9,824
Gross Unrealized Losses	(1,061)	(339)
Available-for-sale securities	1,975,668	2,124,026
Marketable securities	830,824	744,290
Marketable securities	1,144,844	1,379,736
Commercial paper
Marketable Securities [Line Items]
Amortized Cost	51,962	46,931
Gross Unrealized Gains	8	13
Gross Unrealized Losses	(2)	(8)
Available-for-sale securities	51,968	46,936
Marketable securities	51,968	46,936
Marketable securities	0	0
Corporate bonds
Marketable Securities [Line Items]
Amortized Cost	1,525,643	1,628,462
Gross Unrealized Gains	6,535	9,482
Gross Unrealized Losses	(953)	(262)
Available-for-sale securities	1,531,225	1,637,682
Marketable securities	677,111	607,403
Marketable securities	854,114	1,030,279
Municipal securities
Marketable Securities [Line Items]
Amortized Cost		3,495
Gross Unrealized Gains		0
Gross Unrealized Losses		(6)
Available-for-sale securities		3,489
Marketable securities		0
Marketable securities		3,489
U.S. government agency obligations
Marketable Securities [Line Items]
Amortized Cost	392,396	435,653
Gross Unrealized Gains	185	329
Gross Unrealized Losses	(106)	(63)
Available-for-sale securities	392,475	435,919
Marketable securities	101,745	89,951
Marketable securities	$ 290,730	$ 345,968

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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