Fair Value of Financial Instruments (Tables)

6 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Schedule of Fair Value Measurements of Financial Assets and Liabilities

The following table details the fair value measurements within the fair value hierarchy of the Company’s financial assets and liabilities at June 30, 2019 and December 31, 2018:

	June 30, 2019
	Level 1		Level 2		Level 3		Total
	(in thousands)
Cash equivalents and investments:
Money market funds	$	88,394	$	—	$	—	$	88,394
Commercial paper		—		4,083		—		4,083
Corporate bonds		—		71,877		—		71,877
U.S. Treasury securities		—		714,722		—		714,722
Restricted cash:
Certificates of deposit		—		7,522		—		7,522
Total	$	88,394	$	798,204	$	—	$	886,598

	December 31, 2018
	Level 1		Level 2		Level 3		Total
	(in thousands)
Cash equivalents and investments:
Money market funds	$	1,579	$	—	$	—	$	1,579
Commercial paper		—		8,242		—		8,242
Corporate bonds		—		70,728		—		70,728
U.S. Treasury securities		—		413,241		—		413,241
Restricted cash:
Certificates of deposit		—		5,625		—		5,625
Total	$	1,579	$	497,836	$	—	$	499,415

Summary of Composition of Short and Long Term Investments

The following tables summarize the composition of our short- and long-term investments at June 30, 2019 and December 31, 2018.

	June 30, 2019
	Amortized Cost		Unrealized Gains		Unrealized Losses			Aggregate Fair Value
	(in thousands)
Commercial paper	$	4,083	$	—	$	—		$	4,083
Corporate bonds		71,716		190		(29	)		71,877
U.S. Treasury securities		678,995		788		—			679,783
Total	$	754,794	$	978	$	(29	)	$	755,743

	December 31, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses			Aggregate Fair Value
	(in thousands)
Commercial paper	$	8,256	$	—	$	(14	)	$	8,242
Corporate bonds		70,958		3		(233	)		70,728
U.S. Treasury securities		413,323		56		(138	)		413,241
Total	$	492,537	$	59	$	(385	)	$	492,211

Summary of Contractual Maturities of Short and Long Term Investments

The contractual maturities of short-term and long-term investments held at June 30, 2019 and December 31, 2018 are as follows:

	June 30, 2019				December 31, 2018
	Amortized Cost Basis		Aggregate Fair Value		Amortized Cost Basis		Aggregate Fair Value
	(in thousands)				(in thousands)
Due within one year	$	716,328	$	717,115	$	481,071	$	480,761
Due after 1 year through 2 years		38,466		38,628		11,466		11,450
Total	$	754,794	$	755,743	$	492,537	$	492,211