6. MARKETABLE DEBT SECURITIES

We obtain fair value measurement data for our marketable debt 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. At September 30, 2018, $29.7 million due to us for marketable securities with a September 30, 2018 settlement date, for which cash was received in October 2018, is reflected in prepaid expenses and other current assets on our condensed consolidated balance sheet.

The following tables summarize our marketable debt securities at September 30, 2018 and December 31, 2017, in thousands:

	At September 30, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Certificates of deposit	$	18,800	$	—	$	—		$	18,800
Commercial paper		101,379		—		(3	)		101,376
Corporate notes		324,615		11		(247	)		324,379
U.S. government-sponsored enterprise securities		111,427		—		(88	)		111,339
U.S. treasury securities		349,601		1		(274	)		349,328
Total	$	905,822	$	12	$	(612	)	$	905,222

	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 September 30, 2018, in thousands:

	At September 30, 2018
	Amortized Cost		Fair Value
Less than one year	$	905,822	$	905,222
Greater than one year but less than two years		—		—
Total	$	905,822	$	905,222