Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,819,190	$ 1,961,972
Gross Unrealized Gains	3,006	1,943
Gross Unrealized Losses	(10,560)	(459)
Aggregate Fair Value	1,811,636	1,963,456
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		150,000
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Aggregate Fair Value		150,000
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	42,800	73,829
Gross Unrealized Gains	26	23
Gross Unrealized Losses	(7)	(7)
Aggregate Fair Value	42,819	73,845
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,404,124	1,368,668
Gross Unrealized Gains	1,435	1,840
Gross Unrealized Losses	(10,550)	(378)
Aggregate Fair Value	1,395,009	1,370,130
U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	372,266	369,475
Gross Unrealized Gains	1,545	80
Gross Unrealized Losses	(3)	(74)
Aggregate Fair Value	373,808	369,481
Short-Term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	865,111	1,143,006
Short-Term Marketable Securities \| Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value		150,000
Short-Term Marketable Securities \| Commercial paper
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	42,819	73,845
Short-Term Marketable Securities \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	725,175	753,538
Short-Term Marketable Securities \| U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	97,117	165,623
Long-Term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	946,525	820,450
Long-Term Marketable Securities \| Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value		0
Long-Term Marketable Securities \| Commercial paper
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	0	0
Long-Term Marketable Securities \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	669,834	616,592
Long-Term Marketable Securities \| U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	$ 276,691	$ 203,858

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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