MARKETABLE SECURITIES

3 Months Ended

Mar. 31, 2018

Investments Debt And Equity Securities [Abstract]

4. MARKETABLE SECURITIES

We obtain fair value measurement data for our marketable securities from independent pricing services. We perform validation procedures to ensure the reasonableness of this data. This includes meeting with the independent pricing services to understand the methods and data sources used. Additionally, we perform our own review of prices received from the independent pricing services by comparing these prices to other sources and confirming those securities are trading in active markets.

The following tables summarize our marketable securities at March 31, 2018 and December 31, 2017, in thousands:

	At March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Certificates of deposit	$	33,541	$	—	$	—		$	33,541
Commercial paper		96,539		—		—			96,539
Corporate notes		401,930		5		(900	)		401,035
U.S. government-sponsored enterprise securities		379,692		—		(831	)		378,861
U.S. treasury securities		262,722		—		(335	)		262,387
Total	$	1,174,424	$	5	$	(2,066	)	$	1,172,363

	At December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Certificates of deposit	$	30,200	$	—	$	—		$	30,200
Commercial paper		56,951		—		—			56,951
Corporate notes		373,736		11		(495	)		373,252
U.S. government-sponsored enterprise securities		399,281		—		(983	)		398,298
U.S. treasury securities		200,649		1		(175	)		200,475
Total	$	1,060,817	$	12	$	(1,653	)	$	1,059,176

We classify our debt security investments based on their contractual maturity dates. The following table summarizes our available-for-sale debt securities by contractual maturity, at March 31, 2018, in thousands:

	At March 31, 2018
	Amortized Cost		Fair Value
Less than one year	$	1,173,776	$	1,171,715
Greater than one year but less than two years		648		648
Total	$	1,174,424	$	1,172,363