Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 799,609	$ 1,059,525
Gross Unrealized Gains	956	1
Gross Unrealized Losses	(748)	(4,770)
Aggregate Fair Value	799,817	1,054,756
Unrealized losses included in accumulated other comprehensive loss	293,900
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	13,558	282,996
Gross Unrealized Gains	22	0
Gross Unrealized Losses	0	(50)
Aggregate Fair Value	13,580	282,946
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	710,794	685,653
Gross Unrealized Gains	915	1
Gross Unrealized Losses	(691)	(4,309)
Aggregate Fair Value	711,018	681,345
Available-for-sale marketable securities, continuous unrealized loss position for more than 12 months	(600)
U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	75,257	50,876
Gross Unrealized Gains	19	0
Gross Unrealized Losses	(57)	(404)
Aggregate Fair Value	75,219	50,472
Short-Term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	526,317	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	13,580	282,946
Short-Term Marketable Securities \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	442,522	482,088
Short-Term Marketable Securities \| U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	70,215	50,472
Long-Term Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	273,500	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	268,496	199,257
Long-Term Marketable Securities \| U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Aggregate Fair Value	$ 5,004	$ 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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