Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Marketable Securities [Line Items]
Amortized Cost	$ 1,788,340	$ 1,961,972
Gross Unrealized Gains	13,820	1,943
Gross Unrealized Losses	(387)	(459)
Available-for-sale securities	1,801,773	1,963,456
Short-Term Marketable Securities
Marketable Securities [Line Items]
Available-for-sale securities	800,055	1,143,006
Long-Term Marketable Securities
Marketable Securities [Line Items]
Available-for-sale securities	1,001,718	820,450
Commercial paper
Marketable Securities [Line Items]
Amortized Cost	26,897	73,829
Gross Unrealized Gains	77	23
Gross Unrealized Losses	(1)	(7)
Available-for-sale securities	26,973	73,845
Commercial paper \| Short-Term Marketable Securities
Marketable Securities [Line Items]
Available-for-sale securities	26,973	73,845
Commercial paper \| Long-Term Marketable Securities
Marketable Securities [Line Items]
Available-for-sale securities	0	0
Corporate bonds
Marketable Securities [Line Items]
Amortized Cost	1,399,255	1,368,668
Gross Unrealized Gains	12,877	1,840
Gross Unrealized Losses	(339)	(378)
Available-for-sale securities	1,411,793	1,370,130
Securities in an continuous unrealized loss position for twelve month or longer	1,600
Corporate bonds \| Short-Term Marketable Securities
Marketable Securities [Line Items]
Available-for-sale securities	692,231	753,538
Corporate bonds \| Long-Term Marketable Securities
Marketable Securities [Line Items]
Available-for-sale securities	719,562	616,592
U.S. government agency obligations
Marketable Securities [Line Items]
Amortized Cost	362,188	369,475
Gross Unrealized Gains	866	80
Gross Unrealized Losses	(47)	(74)
Available-for-sale securities	363,007	369,481
U.S. government agency obligations \| Short-Term Marketable Securities
Marketable Securities [Line Items]
Available-for-sale securities	80,851	165,623
U.S. government agency obligations \| Long-Term Marketable Securities
Marketable Securities [Line Items]
Available-for-sale securities	$ 282,156	203,858
Certificates of deposit
Marketable Securities [Line Items]
Amortized Cost		150,000
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Available-for-sale securities		150,000
Certificates of deposit \| Short-Term Marketable Securities
Marketable Securities [Line Items]
Available-for-sale securities		150,000
Certificates of deposit \| Long-Term Marketable Securities
Marketable Securities [Line Items]
Available-for-sale securities		$ 0

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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