Investments (Tables)

9 Months Ended

Sep. 30, 2019

Investments, Debt and Equity Securities [Abstract]

Summary of Investments

Our investments consisted of the following (in thousands):

	As of September 30, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Certificates of deposit	$	3,881	$	27	$	—		$	3,908
Corporate notes and bonds	473,473		1,949		(316		)	475,106
U.S. Treasuries	64,273		18		(48		)	64,243
U.S. Government agencies	156,664		7		(111		)	156,560
Total	$	698,291	$	2,001	$	(475	)	$	699,817

	As of December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Corporate notes and bonds	$	450,097	$	44	$	(1,818	)	$	448,323
U.S. Treasuries	112,783		2		(85		)	112,700
U.S. Government agencies	146,110		—		(442		)	145,668
Total	$	708,990	$	46	$	(2,345	)	$	706,691

Summary of Gross Unrealized Losses and Fair Value of Investments in a Continuous Unrealized Loss Position

The following tables present the gross unrealized losses and related fair values of our investments that have been in a continuous unrealized loss position (in thousands):

	As of September 30, 2019
	Less Than 12 Months					Greater Than 12 Months					Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
Corporate notes and bonds	$	117,902	$	(300	)	$	51,197	$	(16	)	$	169,099	$	(316	)
U.S. Treasuries	—		—			33,972		(48		)	33,972		(48		)
U.S. Government agencies	91,497		(93		)	43,740		(18		)	135,237		(111		)
Total	$	209,399	$	(393	)	$	128,909	$	(82	)	$	338,308	$	(475	)

	As of December 31, 2018
	Less Than 12 Months					Greater Than 12 Months					Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
Corporate notes and bonds	$	420,548	$	(1,817	)	$	1,526	$	(2	)	$	422,074	$	(1,819	)
U.S. Treasuries	105,525		(85		)	—		—			105,525		(85		)
U.S. Government agencies	137,416		(441		)	—		—			137,416		(441		)
Total	$	663,489	$	(2,343	)	$	1,526	$	(2	)	$	665,015	$	(2,345	)

Summary of Contractual Maturities of Investments

The following table summarizes the contractual maturities of our investments at September 30, 2019 (in thousands):

	Amortized Cost		Fair Value
Due within one year	$	328,664	$	328,862
Due within one to three years	369,627		370,955
Total	$	698,291	$	699,817