MARKETABLE DEBT SECURITIES

3 Months Ended

Mar. 31, 2019

Investments Debt And Equity Securities [Abstract]

7. 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. We did not record any impairment charges related to our marketable debt securities during the three months ended March 31, 2019 or 2018.

The following tables summarize our marketable debt securities at March 31, 2019 and December 31, 2018, in thousands:

	At March 31, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Certificates of deposit	$	3,000	$	—	$	—		$	3,000
Commercial paper		34,394		—		—			34,394
Corporate notes		113,281		1		(29	)		113,253
U.S. government-sponsored enterprise securities		2,496		—		—			2,496
U.S. treasury securities		896,699		25		(58	)		896,666
Total	$	1,049,870	$	26	$	(87	)	$	1,049,809

	At December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Certificates of deposit	$	8,951	$	—	$	—		$	8,951
Commercial paper		57,197		—		—			57,197
Corporate notes		232,695		—		(285	)		232,410
U.S. government-sponsored enterprise securities		39,031		—		(13	)		39,018
U.S. treasury securities		546,631		1		(124	)		546,508
Total	$	884,505	$	1	$	(422	)	$	884,084

The fair values of our marketable debt securities by classification in the condensed consolidated balance sheets were as follows, in thousands:

	At March 31, 2019		At December 31, 2018
Cash and cash equivalents	$	530,231	$	221,281
Marketable debt securities		519,578		662,803
Total	$	1,049,809	$	884,084

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, 2019, in thousands:

	At March 31, 2019
	Amortized Cost		Fair Value
Less than one year	$	1,049,870	$	1,049,809
Greater than one year but less than two years		—		—
Total	$	1,049,870	$	1,049,809