Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 458,191	$ 737,090
Unrealized Gains	740	33
Unrealized Losses	(164)	(2,204)
Fair Value	458,767	734,919
Current | Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		960
Unrealized Gains		0
Unrealized Losses		(4)
Fair Value		956
Current | U.S. Treasuries
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	160,981	231,101
Unrealized Gains	122	7
Unrealized Losses	(32)	(228)
Fair Value	161,071	230,880
Current | Government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	58,093	75,335
Unrealized Gains	15	0
Unrealized Losses	(47)	(121)
Fair Value	58,061	75,214
Current | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	150,851	208,233
Unrealized Gains	241	0
Unrealized Losses	(36)	(483)
Fair Value	151,056	207,750
Non-current | U.S. Treasuries
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		12,202
Unrealized Gains		4
Unrealized Losses		(125)
Fair Value		12,081
Non-current | Government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,594	70,177
Unrealized Gains	15	10
Unrealized Losses	(24)	(188)
Fair Value	21,585	69,999
Non-current | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	66,672	139,082
Unrealized Gains	347	12
Unrealized Losses	(25)	(1,055)
Fair Value	$ 66,994	$ 138,039