3. Cash Equivalents and Marketable Securities

The following tables summarize the estimated fair value of the Company’s cash equivalents and marketable securities and the gross unrealized gains and losses (in thousands):

	March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Cash equivalents:
Money market funds	$	1,370	$	—	$	—		$	1,370
Reverse repurchase agreements		6,250		—		—			6,250
Total cash equivalents		7,620		—		—			7,620

Marketable securities:
U.S. government securities		32,813		—		(98	)		32,715
Total marketable securities	$	32,813	$	—	$	(98	)	$	32,715

	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Cash equivalents:
Money market funds	$	1,674	$	—	$	—		$	1,674
Reverse repurchase agreements		7,250		—		—			7,250
Total cash equivalents		8,924		—		—			8,924

Marketable securities:
U.S. treasury securities		1,502		—		(1	)		1,501
U.S. government securities		36,082		—		(101	)		35,981
Total marketable securities	$	37,584	$	—	$	(102	)	$	37,482

The reverse repurchase agreements are settled in cash nightly, and as such are classified as cash equivalents.

As of March 31, 2018 and December 31, 2017, all debt securities with an unrealized loss position have been in a loss position for less than one year. The aggregate fair value of debt securities in an unrealized loss position as of March 31, 2018 and December 31, 2017 were $32.7 million and $37.5 million, respectively, with no individual securities in a significant unrealized loss position. The Company evaluated its securities for other-than-temporary impairment and considered the decline in market value for the securities to be primarily attributable to current economic and market conditions and would not be required to sell the securities before recovery of the amortized cost basis. Based on this analysis, these marketable securities were not considered to be other-than-temporarily impaired as of March 31, 2018 and December 31, 2017.

The following tables summarizes the contractual maturities of the Company’s marketable securities at estimated fair value (in thousands):

	March 31,		December 31,
	2018		2017
Due in one year or less	$	32,715	$	34,498
Due in 1 - 2 years		—		2,984
Total marketable securities	$	32,715	$	37,482

The Company may sell investments at any time for use in current operations even if they have not yet reached maturity. As a result, the Company classifies marketable securities, including securities with maturities beyond twelve months as current assets.