3. Cash Equivalents and Marketable Securities

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

	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

	December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Cash equivalents:
Money market funds	$	4,584	$	—	$	—		$	4,584
Reverse repurchase agreements		39,250		—		—			39,250
Total cash equivalents		43,834		—		—			43,834
Marketable securities:
U.S. government securities		15,758		—		(4	)		15,754
Total marketable securities	$	15,758	$	—	$	(4	)	$	15,754

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

As of December 31, 2017 and 2016, 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 December 31, 2017 and 2016 were $37.5 million and $15.8 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 December 31, 2017 and 2016.

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

	December 31,
	2017		2016
Due in one year or less	$	34,498	$	15,754
Due in 1 - 2 years		2,984		—
Total marketable securities	$	37,482	$	15,754

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.