Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,551,423	$ 1,059,525
Gross Unrealized Gains	1,527	1
Gross Unrealized Losses	(1,112)	(4,770)
Aggregate Fair Value	1,551,838	1,054,756
Unrealized losses included in accumulated other comprehensive loss	125,400
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		40,000
Gross Unrealized Gains		0
Gross Unrealized Losses		(7)
Aggregate Fair Value		39,993
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	78,454	282,996
Gross Unrealized Gains	20	0
Gross Unrealized Losses	(38)	(50)
Aggregate Fair Value	78,436	282,946
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,286,133	685,653
Gross Unrealized Gains	1,491	1
Gross Unrealized Losses	(878)	(4,309)
Aggregate Fair Value	1,286,746	681,345
Available-for-sale marketable securities, continuous unrealized loss position for more than 12 months	(100)
U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	186,836	50,876
Gross Unrealized Gains	16	0
Gross Unrealized Losses	(196)	(404)
Aggregate Fair Value	186,656	50,472
Short-Term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	831,508	855,499
Short-Term Marketable Securities \| Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value		39,993
Short-Term Marketable Securities \| Commercial paper
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	78,436	282,946
Short-Term Marketable Securities \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	638,604	482,088
Short-Term Marketable Securities \| U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	114,468	50,472
Long-Term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	720,330	199,257
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	648,142	199,257
Long-Term Marketable Securities \| U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	$ 72,188	$ 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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