3. Cash Equivalents and Investments

Cash equivalents are highly liquid investments that are readily convertible into cash with original maturities of three months or less when purchased. Investments consist of securities with original maturities greater than 90 days when purchased. The Company classifies these investments as available-for-sale and records them at fair value in the accompanying condensed consolidated balance sheets. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment.

Cash equivalents and investments, available-for-sale, consisted of the following at June 30, 2018 and December 31, 2017 (in thousands):


	Average		Amortized		Unrealized		Unrealized		Fair
June 30, 2018	Maturity		Cost		Gain		Losses		Value
Cash equivalents:
Money market funds			$	117,751	$	—	$	—	$	117,751
Investments, available-for-sale:
U.S. treasury obligations		282 Days		499,281		7		(388)		498,900
Total			$	617,032	$	7	$	(388)	$	616,651


	Average		Amortized		Unrealized		Unrealized		Fair
December 31, 2017	Maturity		Cost		Gain		Losses		Value
Cash equivalents:
Money market funds			$	400,304	$	—	$	—	$	400,304
Investments, available-for-sale:
U.S. treasury obligations		348 Days		273,321		—		(269)		273,052
Total			$	673,625	$	—	$	(269)	$	673,356

Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the three and six months ended June 30, 2018 and 2017, there were no realized gains or losses on sales of investments, and no investments were adjusted for other than temporary declines in fair value.

At June 30, 2018, the Company held 48 securities that were in an unrealized loss position. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of June 30, 2018 was $412.6 million. As of June 30, 2018, there were no securities held by the Company in an unrealized loss position for more than twelve months. The Company has the intent and ability to hold such securities until recovery. The Company determined that there was no material change in the credit risk of the above investments. As a result, the Company determined it did not hold any investments with an other-than-temporary impairment as of June 30, 2018.